To be fair, I think the git vs. mercurial advocates are few and far between compared to the git vs. centralized advocates. However, the reasons are easy to summarize:

Git is version control for programmers. Mercurial is version control for enterprises. Centralized was an adequate first try at inventing version control, especially considering it was designed before the personal computer revolution.

What I mean by version control for programmers is that programmers in general favor flexibility over ease of learning. After all, we're willing to spend years learning esoteric languages in order to have the flexibility to make computers do what the untrained cannot. Git gives programmers the flexibility to use it however they please, at the expense of it taking longer to learn how to use that flexibility safely. It allows restrictions to be put in place to enforce policies, but doesn't come that way out of the box. Note I said ease of learning rather than ease of use. Once you learn it, git is as easy to use as any other VCS, and often easier due to increased speed and features.

Some programmers learn enough to do what they want, then resist learning new ways to do it. Enterprises hire and employ many of these people, so they want any changes in tools they use to have a certain degree of familiarity. Enterprises also want their programmers to have enough flexibility to do their jobs, but not so much as to make training or initial migration difficult. This is where mercurial fits in. It has most of the power of git, but a somewhat easier migration path.

I don't think it's fair to say git is only popular because of the hype, or Linus' endorsement. That probably accounts for many people trying it, but they stick with it and promote it because it works well for them, pure and simple.