Unless you customize it, Firefox thrusts ads and/or click bait in your face when you open a new window. I forgot if Chrome does this, for I’ve already de-turded Chrome.
Firefox used to constantly beg me to use “Pocket”, which I’ve never had any interest in. But I haven’t seen that in a long time.
So there are a few factors that could deter people from *starting* to use Firefox, but they’re not major ones. Another is inertia: if one is used to using another browser, and has installed extensions, made bookmarks, etc., they may not want to switch to Ff.
I use Chrome primarily, but with all the bad news coming out about Ff, I’ll likely abandon it for Ff.