The consumer hardware duopoly of Apple and Samsung are the only ones who seem to actually drive manufacturing trends. There are also tons of devices being made for the Chinese market, but you can't buy those because they're usually locked up in supplier agreements and honestly they don't meet "flagship" specs for display quality.
Component suppliers, true we-make-parts manufacturers, are not really trying to influence the big picture so much as make sure they are running their manufacturing lines at capacity. And if they are building panels on spec for open market sales, they are going to build >6" displays because it's a higher probability they'll actually sell at volume.
So supply chains behave like ecosystems.
In the natural world we see insects and animals develop things like bright plumage and big horns because the animal before them was successful in doing the same thing. This behavior can go on for a long time too. Then an asteroid hits (tantamount to bad economic times) and the fast moving generalists seem to succeed better than the highly adapted specialist.