Many reasons. Model 3 is to big. The Model 3 is missing many features needed, like inductive charging, automatic doors. And has lots of things not needed, like a steering wheel column. Model 3 is build on a much older architecture, even the upgraded versions. Model 3 still uses traditional car wiring.

With all the changes you would have to do to M3, its basically a new car.

This Robotaxi will have all the drive-by-wire architecture of the Cybertruck. The new electronics architecture and Ethernet bus. And things like wireless charging.