A quick comparison of iPhone 6 vs 6+ preorder supply (from 9to5Mac):
and preorder demand (from data in AT&T threads):
Demand for the 6+ was about 3 times that of the 6, but only 12% of phones delivered were iPhone 6's. Looks like Apple either really underestimated the demand for 6+ or there really is a serious manufacturing issue.

and preorder demand (from data in AT&T threads):

Demand for the 6+ was about 3 times that of the 6, but only 12% of phones delivered were iPhone 6's. Looks like Apple either really underestimated the demand for 6+ or there really is a serious manufacturing issue.