This is older than the current smartphone duopoly; IIRC, with a few exceptions like the N900 and the Palm Treo, cell phones were always very locked down. It probably came initially from the phone companies being control freaks, and wanting to restrict which devices (even mechanical ones with no electrical or radio interface) could be attached to their phone system.
> N900 and the Palm Treo
Performance-wise, a modern iPhone is more closely comparable to room-sized (building-sized?) supercomputer of that era.