It's really hard to clean up media. You have to go into every single chat and from there go about deleting stuff. At least they finally added a "select all" option in there recently.
So the size of it just grows and grows and grows until it's using all the space on your phone. Not a good fit for non-technical types.
Secondly, no web view. There is the desktop app yes, which is flaky, slow and wants to update every day or two.
I just can't see average people putting up with those inconveniences and that's just a couple of them.
To be fair I've met plenty of non-techie types whose phones were "full" of stuff from WhatsApp or photos that had already been backed up, because the idea they could clear their local storage would never cross their minds. I've seen people buy new phones instead of clearing their cache.