Because the so-called Purple Haze going around now was released by one specific breeding outfit.
Damn, let me go into my genetics collection and see what I've got... How's about a 4-way double Haze cross, and a Purple Star male who I keep around just to laugh at him... Give me a couple generations and I will supply you with a product that tastes similar to, but isn't quite, tea... I mean, Haze (any Douglas Adams fans out there, lol?)... and is purple to boot. And yes, it will be mediocre, and look like a sativa, because Purple Star sucks, and both have sativa heritage.

-DISCLAIMER- I am not giving out any seeds so don't ask! This is hypothetical!!!

Anyway... only one breeder seems to have been dodgy enough to want to capitalize on the STUPID purple haze buzzword. That's why what you got was so consistent.

It's easy enough to breed purple buds into a haze line, but why on EARTH would you bother, unless -OMG!!! Could it be?- you are just out to score a bunch of dough?