Technological stagnation
At least in my pool of contacts, I think the reason is simply phones have reached the peak of what they can offer.
A phone is like a Swiss Army pen knife - lots of tools but all pretty rubbish because of space limitations. e.g. You are stuck with a tiny 4 mm lens with a 4 mm focal length, and no amount of fancy electronics can fix that. Ditto antennas, user interfaces, batteries, speakers, buttons, and so on.
