There was simply no reason for Orochimaru to forget a convenient forbidden technique like the Edo Tensei.
Coincidentally, this jutsu, which desecrates the dead and is both inhumane and unethical, was invented by the master of forbidden techniques himself, the Second Hokage. Yet, during his lifetime, Tobirama failed to implement defensive measures to prevent his own summoning after death. As the saying goes, what goes around comes around; good and evil deeds eventually receive their due. Thus, whenever someone uses Edo Tensei, the Second Hokage is invariably among the resurrected dead being manipulated. One could say he reaped what he sowed.
Strictly speaking, there are three versions of Edo Tensei. The version developed by the Second Hokage during his lifetime could be considered the alpha version. Orochimaru's slightly improved version, built upon the Second's foundation, is the beta version. Finally, there's the new version developed by Yakushi Kabuto based on Orochimaru's, featuring improvements like reduced burden on the caster, enhanced control, and a vastly increased number of possible resurrections – this could be called the public release version.
Due to Orochimaru's focus being diverted by certain... suggestions from Habara over the past few years, his research progress on Edo Tensei lagged significantly behind the original timeline. Consequently, his mastery of the jutsu certainly hadn't reached the level of "widespread mass application," meaning using it still placed a considerable strain on him.
For Orochimaru, this risk wasn't unbearable, but the problem lay in finding the right balance... The First Hokage was undoubtedly the God of Shinobi. However, if Orochimaru completely suppressed his consciousness, resurrecting him merely as a mindless puppet, Orochimaru had reason to doubt such a version of the First could defeat the Rinnegan.
The logic was simple: Orochimaru had to give the Rinnegan some respect. Remove the true god's mind, and the true god becomes merely a false god. A false god versus another false god, the mindless versus the mindful – what chance of victory was there?
Yet, if he completely released the First Hokage's consciousness, Orochimaru worried the resurrected First might instantly break free from his control. Imagine an Edo Tensei version of the First Hokage roaming the shinobi world... Although the resurrected First couldn't reach the full strength of his living self, his "survivability" arguably surpassed that of the living First. It was hard to say which scenario would be more troublesome.
Therefore, Orochimaru sought a middle ground where the First Hokage wouldn't break free but would retain enough power to defeat the Rinnegan. Finding this balance was incredibly difficult... or perhaps, fundamentally impossible.
Compared to the First Hokage, the Second Hokage wasn't worth agonizing over. Tobirama was just being resurrected as a bonus, thrown in like free pickles when buying ten pounds of steamed buns.
After much deliberation and failing to find a foolproof method, Orochimaru finally made a rational yet risky decision... He would use weak "sacrifices" to summon both Hokage, preserve their consciousness, and simply talk to them first. Have a frank discussion.
Under the threat of the Rinnegan, this was a critical juncture for the survival of the shinobi world. Shouldn't the two Hokage step forward at such a time? With great power comes great responsibility, right? Wasn't protecting the shinobi world the First Hokage's natural duty?
What? The First Hokage was long dead? Does death absolve one of responsibility? This wasn't just lifetime accountability; it was eternal-damnation-no-escape accountability.
Orochimaru made up his mind, but he didn't act immediately. Pulling such stunts within Amegakure, right under the "leader's" nose, was still a bit too stimulating.
After finding an opportunity to leave the Land of Rain, Orochimaru traveled all the way to the desolate highlands of the Land of Earth. After cautiously preparing a hidden underground location, he finally prepared to awaken the two Hokage... As an aside, several days had passed since Manda was "repurposed," yet Orochimaru remained blissfully unaware.
It couldn't be helped. When a scientist becomes engrossed in research, they tend to neglect sleep and meals, completely unaffected by external matters.
With all preparations complete, Orochimaru summoned the two coffins. As he slowly formed the hand seals, the two Hokage within the coffins gradually opened their eyes.
People truly cannot be generalized, not even blood brothers. This was evident from the immediate reactions of the First and Second Hokage upon awakening.
The Second Hokage awoke, his gaze sweeping around, instantly becoming deep and extremely vigilant... Clearly, he had already deduced the nature of his "resurrection."
The First Hokage was entirely different. Upon waking, he first awkwardly shifted his incredibly stiff body, causing his armor to creak noisily. Then, he vigorously shook his numb head, even tilting it and slapping the side of his head with his hand like someone trying to get water out of their ear after swimming. Only after finishing this did he seem to notice something, staring intently at the hand he had just raised.
"This is... something's wrong... Theoretically, I should have died long ago."
The First Hokage, despite living a full life and being buried for many years, still retained his original spirit – a pristine innocence untainted by excessive knowledge.
"Big brother, we are indeed dead. The reason we've been summoned to the present world, if I'm not mistaken, is due to the Edo Tensei technique I developed. However, it's a forbidden technique prohibited for use by ordinary ninja..." the Second Hokage, Tobirama Senju, immediately explained to his brother. The scene was almost identical to how they interacted when alive.
The one with fists contributes fists; the one with brains contributes brains. Relying on the brain's fists or the fists' brain was best avoided if possible.
"Tobirama? So you died too? Edo Tensei..." The First Hokage understood immediately upon hearing the name. He instantly shook his head. "Tobirama, ah, I told you long ago not to develop such pointless forbidden techniques. Now you've truly woven a cocoon around yourself, haven't you?"
"Brother, how can you call it a pointless technique? You..."
"Haha, that's not what I meant... What era is it now?" Hashirama interrupted, changing the subject.
As a "mere mortal" like the Second Hokage, constantly trailing behind a cheat-code-level individual like the First, what else could he do but continuously develop forbidden techniques to avoid being left too far behind? Although, even after developing so many forbidden techniques, he was still left just as far behind.
"Now? The dead don't have an accurate sense of time. I've also been dead for many years, but... if nothing unexpected happened, this should be the era when Sarutobi leads Konoha," the Second Hokage replied, a rare look of reminiscence and nostalgia gracing his features.
At this point, Orochimaru, unable to tolerate the two brothers casually chatting immediately after waking up, finally cut into their conversation. "Incorrect. Sensei Sarutobi is also dead. Konoha has entered a new era... an era where the influence of you two has almost completely vanished."
This Orochimaru fellow truly understood the art of conversation – specifically, how to stir up trouble.