You cant stream it on data connection without obliterating your data cap and battery.

You cant simply load FLACs onto your phone it kills the free storage in the blink of an eye. Try loading 1000 FLACs vs MP3s.

And moreover, there is a debatable gain or quality when you are on mobile with mobile gears/earphones.

MP3s fit in the middle of all restrictions.

I like FLACs, of course, but I can see why people just prefer MP3s