Mainly because it's look and feel. I'm not a "power user". Chrome just looks lightweight and feels fast. The only browser that is similar in these aspects is Safari.
Firefox looks bad. For example:
- the spacing between the home button and the search bar
- why there's a line at the top of the current tab? The way Chrome distinguishes the current tab looks "better""
- the "back", "forward" and "refresh" buttons look big