Is it that much faster? (And if so, is that just because it's written in C, or due to some other optimizations?) Ability to eventually do load balancing? Having lots of mod_anything you could possibly want here?
EDIT: I guess my real question is, what exactly does Apache do differently that makes it so widespread/well used?
Apache is free and grabbed marketshare ages ago. Now there's sufficient expertise and proven reliability (ie, reputation) that it's an established player in the webserver market and it's not going anywhere for a long time.
I might say that your question is wrong. It isn't "Why do people choose Apache?" It's "Why wouldn't people choose Apache?" That's the power of an established market presence and a long-term dominance of a particular segment. :)
In other words, I think it has very little to do with the technical aspects of the products involved.