Because Purple Haze doesnt exist anymore. It may have even been a myth that it existed in the first place. If a dealer tells you that the weed youre buying is Purple Haze, then its a load of bollocks. They just get some purple weed (can be done by lowering temperatures in flowering to make the buds purple - my two male lowryders outside have started to turn purple) and give it the name Purple Haze to get you to come back.
Anyways, my favourite is Durban Poison. Not the strongest out there, but I just love the pure sativa high.