Quote Originally Posted by The12lber
Because anyone who would be in the market for that kind of thing could just buy themselves a more effective and equally concealable MP5K.

http://youtube.com/watch?v=8oLcHkCcz_c

This actually has a pretty obvious market. The barrel is only 2 inches shorter than the M4's. However, it uses polygonal rifling which produces higher muzzle velocities (better gas seal behind the bullet) than conventional rifling techniques to make up for this. The weapon however is still much smaller than the M4, 29.8 in (757 mm) (stock retracted) on the M4 compared to 20 inches for the Magpul PDR. If you're going to lose lethality with the shorter barrel... you might as well make it truly compact.
Honestly, I'd rather own both. MP5K is great for government work, but that folder is based on a glock, meaning there would be an affordable, available civilian version, rather than a still super expensive H&K or comparable knockoff.