Probably because Z75 is a silly chipset that is made irrelevant by Z77, much like how P67 was made irrelevant by Z68, although in the case of P67vsZ68, Z68 was several months late to the party giving P67 its own legitimate 15 minutes of fame.
It might just be that the cost savings in implementing Z75 might simply be outweighed by Z77's ability to outsell it, as well as consolidating support. Basically it might just be a situation where the cost savings really wouldn't or couldn't really be passed on to the consumer and thus just not worth it in the interests of a brand name like ASUS. IIRC the cost difference in the two chipsets is something like $8, and who knows, maybe ASUS is big enough that they can get an even better deal that truly trivializes Z75