The market for "I want it for the lower only" would be fairly limited - there would be some who would do it, but not enough to justify an entire product line.
A better alternative would have been to offer the rollmark for the LE6933.
Maybe as an "OEM" model with a pistol receiver extension, too - but AR pistols have never really be Colt's MO, but then again, neither have the OEM models...
~Augee