My guess is that EE is an extremely new mutation (its not really a tail type but whatever). It's very possible that breeders are working with a select group of bettas that all just happen to be lavender/blueish.
My EE is purple (he's the one in my user photo) but I actually really wanted a purple betta and an EE so it worked out perfectly. I have started to see some variety in the EE betta. I've started seeing some gorgeous white and yellow ones. I'm sure with time we'll start to see some other colors.
When betta splendens first came on to the market, all we had was blue and red veiltails and now look at all the varieties.
Just Keep Swimming
your fish are your responsibility; there are no excuses to why you cannot provide for your fish.
Bettas : Stella, Sebastian
Nerite Snail: Cedric