I think as an esteemed poster here used to say...."it's the batteries stupid"
They probably didn't have battery capacity till recently.
That is the advantage of building at Nevada, they can use 2170 cells if they don't need those cells for 3/Y.
Up to now, they have needed those cells for 3/Y.
My final thought is the the curved glass windscreen might be tricky to ramp and ship,
However, a semi load could carry glass windscreens to build more semis than could be built by a semi load of batteries,.
Ideally they would want to make the glass windscreen and the same location as the final semi factory.
I have no idea what is involved in making a curved glass windscreen of that size, but the fundamental process is probably no different to a regular car windscreen.
Austin might eventually be cranking out 4680s in high volumes, have the ability to make the motors and the glass windscreen.
But for now, focusing on Cybertruck at Austin seems sensible,.