Constantinople is in the hands of the Ottoman Empire, directly cutting off Russia's connection between the Black Sea and the Mediterranean.
Any country wishing to aid Russia can only transport supplies via the more northerly Baltic Sea.
However, the Baltic Sea is far from the main battlefield in the Balkans, and compared to directly transporting supplies through the Strait of Constantinople, the wasted time could even alter the situation on the battlefield.
Fortunately, Russia was meticulously prepared before launching the war, and at least in the short term, it didn't lack weaponry.
This is also a credit to the changes brought to Russia by the reforms of Alexander II. In the last Russian-Turkish War, namely the Crimean War, Russia's industrial foundation was extremely poor, and the war ultimately ended in failure.
Conversely, in this war, Russia has established a certain industrial foundation through reforms, which is an extremely significant change for Russia.
