It’s because of a vicious circle. Chrome beat Firefox a long time ago as the IE killer browser (before it became the resource killer it is today) and as such it became both a user and developer darling.
So it’s the browser that gets the most attention and support from devs and in turn it’s a safe choice for users. Not to mention developers are used to the excellent developer tools in Chrome and though others have similar tools, they are still playing catch-up.
Nowadays just about everyone uses Webkit (or Chromium engine altogether) under the hood to make sure they benefit from the baseline support.
As a developer I love Opera myself but it’s still hard to part with Chrome on account of its minimalist defaults (UI wise) and habits built around developer tools.