Question

Short answer of why democracy is a most prevalent form of government

Answer 1

Probably not because it is so equitable, open and problem free. There are no other systems that work any better in the long run, and most others are substantially worse.
government that operates for millions of people will have many problems, usually around the oppression of one or several groups. In democracies, though, each group should be able at the very least to voice opposition peacefully and without threat of retribution. That's how large groups of people can become aware of and possibly sensitive to the needs of others.