Kastello In this case, #3 is clearly not the reason, as the removable back already did this.
I’m extremely skeptical of #2. USB has two conductors. There is not going to be a significant difference between a good connector and a soldered connection. Just doesn’t track on multiple levels.
#3 Has some merit, but again, since they already made the back removable, there is no real need for an external case, just a connector vs soldered in.
This is one of many decisions that have been forced on consumers due to undue market influence. I truly feel that many consumers would choose a thicker phone with a removable battery in a heartbeat, for many reasons.