Why no WR pancake lenses?

I'm deliberating the purchase of a K-5 or K-30, one of the major reasons being the environmental sealing. I don't have any lenses, but keeping the compact form is very important to me. The 21 mm., even though expensive, would be the one I'd choose. But this doesn't have WR, and the same goes for all the other Pentax pancakes.
Is there a reason for this lack of WR pancakes? Thanks.