It's pretty simple - for regular users Chrome is faster, has more features, uses less power, and has better website support. It is also aggressively marketed by the largest advertising company on the planet.

It wasn't always this way. For example, FF beat Chrome in battery life until a few years ago, and FF extensions used to be superior in functionality Chrome extensions. Chrome eventually caught up in the few areas they were seriously lagging in, or Firefox shot itself in the foot trying to morph itself into a copy of Chrome (eg. multiprocess architecture, WebExtensions).

Today there is little reason to use any browser other than Chrome / Chromium or Safari (which is still superior on Mac / iOS in a few aspects).