Space, as a connector is slightly bigger than a soldered connection, and the battery itself has to be slightly bigger and stronger too to be safe to transport and handle without being protected by the phone’s case.
Water resistance; it’s far easier to make a phone’s case waterproof by just glueing the whole thing shut than having to use seals and gaskets and such to make it possible to open and close it at will.
That’s pretty much how it used to work.
The main reasons why it changed are:
Space, as a connector is slightly bigger than a soldered connection, and the battery itself has to be slightly bigger and stronger too to be safe to transport and handle without being protected by the phone’s case.
Water resistance; it’s far easier to make a phone’s case waterproof by just glueing the whole thing shut than having to use seals and gaskets and such to make it possible to open and close it at will.