MMH5.5 - Heroes of Might and Magic 5.5

If I understand correctly, the mod also increases the average game length. Why was this done? Were the games too short before?

The mod doesn't directly increase the game length, but it supports longer games. That is, there are still settings for games of different lengths, but a longer game is preferred due to balance. In a short game, it may not have time to fully reveal its potential.