It's not a stupid question, some people just don't know.

Mainly it's because:

1. Chromium holds too much market share which is bad for the health of the Web.
2. Chromium is controlled by Google which is concerning because they have been known to plant trackers even in software that shouldn't have them.
3. Chromium is inherently less secure, it contains features that might seem nice but are extremely risk to give access to websites i.e. letting websites access Bluetooth.

There are probably plenty more reasons but these are the big ones, and of coarse this is a simplification, in reality things are always a bit more complicated.