The goal is to have something that isn't limited to a device. This lessens the amount of code that is locked to one make, or even worse, one model. It also reduces the chance of security issues, since one company's EncFS implementation may be brain dead, while another's is quite up to par with security.
Same with device encryption. If each maker did its own thing with regards to encrypting
The more security in base Android, the better. This at least provides a baseline that we know is good. For example, we know any Android 4.x and 5.x device offers encryption of
Limiting code to one make/model isn't good. For example, there are features of my old Atrix 2 which no new phones have, and likely would never have.