Maybe you planted all purple varieties or maybe just the purples germinated??

Seems like I spoke too soon, first a few crimson, then a stay blue flower here and there, and now some lovely white ones.

So I suppose a better question would be, why do the purple ones grow so much faster and flower so much more prolifically then other colors?

The I. purpureas are incredibly prolific here as well. I guess they are just a hardier variety and suited to our zone. Maybe more closely related to the native varieties? They also reseed like crazy and I pull them up by the trashbag full every spring and summer. My Japanese MG's are much slower to mature, smaller and don't reseed. I wish it was the other way around.