the crystals are less efficient requiring larger pump diodes as well as the crystals being made of more costly materials - plus it's a fairly new technology so it's still quite expensive
I guess there are 2 reasons.. its fundamentally more difficult to achieve, and the market is much smaller. Blue results from a minor Nd:YAG line (946) being doubled, while green is fromt the primary 1064 nm line. Also, doubling is less efficient, making it all that more difficult to build a blue DPSS.