1) their security isn't good just because of their scale/size (that begins to seem more and more like a false-assumption nowadays)
2) migrating your email to a new provider is quite difficult (consider that the average person will have just 1 - or 2 - email accounts and they link EVERYTHING to it)
3) the price of ads/convenience is no longer worth it. I'm assuming at least a sizable minority of internet users are using ad-blockers these days. They can't get your eyeballs, so they package and sell your data. Granted, you can probably now get the same (raw) data on the black market by paying a fraction in bitcoin and you'll get to see those billions of emails telling people someone attacked their farm in farmville from 2009
Lastly (and I really hope this happens), Yahoo implodes/collapses (cause the average Joe won't migrate willingly) and leaves a vacuum for their 500+ million email users. Hopefully the smaller providers (Proton, Migadu, Posteo, Tuta, etc.) get at least 10% of these users and the email-cartel is broken (somewhat).