This is a really, really good question. Some things:
- It's fast, which we all love.
- It mostly does painless merges, again we love.
- It's not actually that hard to 'get going' - and I think that a lot of people are totally overconfident in their skills.
- It's free
- It's open source
- Linus Torvalds created the critical mass for Linux
- GitHub
- Lack of truly efficient alternatives
- It technically 'works' i.e. does what it's supposed to do, relatively robustly.
- It's designed for open source, which is a novel concept.
Personally, I'm scared of git from an organizational perspective, and I do not feel that we have reconciled the reality of how much pain it can cause to people, newer teams, the less familiar etc..
If people would pay for software I wonder if a great alternative could come to the fore because without it, we have to rely on volunteers and that's an ugly industry problem.