Ultimately, it's because the power source of horse-drawn carriages, the horses, have too many limitations. After all, they're animals, subject to birth, aging, sickness, and death, along with a host of other issues.
Even the army's well-trained warhorses can't endure prolonged combat. Not to mention the ordinary horses used for transportation, which frequently encounter problems while working.
Even when several horses pull a carriage, transporting anything with significant weight is greatly restricted.
Weather and temperature changes, as well as road conditions, also affect the transportation of carriages, which in turn impacts economic development to some degree.
While cars also have various issues, they generally focus on the engine, specifically the so-called stability problems.
As long as the engine's stability issue is resolved, cars can transport freely as long as there's enough fuel, traveling as long and as far as desired.
