The same reason why people don't use Internet Explorer on a PC: More options, features and better speed. Different browsers on Android have different engines, unlike on iOS where everything is forced to use the Safari engine.
A few examples of differences:
1. Opera - has an engine with turbo mode option, allowing for server side compression making you use less data and less time to render a webpage than other browsers
2. Chrome- uses the Google Chromium engine, so its lean fast and allows for sync between your device and PC Chrome app. You can even see what tabs are currently open on your PC if you leave it running at home