Why not use the 97% efficient permanent magnet motor in the rear too?

When designing the Model 3, Tesla made the decision to make them affordable, but also not too fast. All Model 3s have a 270hp permanent magnet motor in the back, which has a significantly slower acceleration at low RPM than the induction motors in the Model S and X.

As Jason from Engineering Explained explains, the permanent magnet motor in the Model 3 has significant torque ripple at low RPM, so Tesla had to reduce its power to avoid uncomfortable intermittent acceleration.

The Model 3 AWD and Performance edition have a small induction motor in the front, which helps with low RPM acceleration, achieving 3.2s 0-60 mph.

This is why Tesla decided to replace only the front motor in Model S and X, keeping the extremely powerful induction motor in the back, which allows accelerations of up to 2.2s 0-60 mph in the Model S and about a half a second worse in the Model X.