Much the same reason as no current commercial aircraft is supersonic, drag/cost climbs substantially and the aerodynamics becomes much more complex given the need to take off subsonic and fly supersonic. Notably This drone gives an example of the challenges and constraints of getting an external payload above mach one and separating it cleanly. the listed 5000kg weight also suggests how much bigger the launch platform would have to be (Pegasus is ~20000kg).
There are limited gains for doing this, going from 200 m/s to mach2 / 600 m/s is only a small fraction of the needed final velocity of 7400 m/s. So your overall rocket size shrinks by a small amount while the size and complexity of your launch platform climbs massively.
The current air launch platforms make sense because they use cheap(ish) and reliable aircraft to launch from, and have the ferry range to find clear weather/safe ground track for a launch to avoid scrubs due weather or obstacles (for customers where not hitting launch windows causes costs in some way).