Because a some years back and for many years it was slower. Chrome came as a default for many devices and many sites css where also optimized for it, and soon

Because a some years back and for many years it was slower. Chrome came as a default for many devices and many sites css where also optimized for it, and soon other browsers adopted its engine. Firefox had a niche of better browser addons but Mozilla leadership killed it. For mobile it is even more handicapped. Disclaimer: Longtime FF user who feels like a good friend is gone