Lack of aftermarket parts support is the key reason. My Friend has a TM Uzi, getting parts and a gun doc dumb enough to work on it is a challenge.
Low fps means it's harder to import in then the GBB SMG's. External Build quality on AEP's Generally are on the poor side, same with the small battery unless you wire to take external.
If I could get spare internals, and mechboxes without having to wait for Tokyo Marui to respond etc I would be interested. Always wanted a Skorpian or Uzi automatic pistol.