Atmel's SAM C21 not available?
The C21, AFAIK, is a new chip, so it's likely they have not been distributed yet. 

Setting up the distribution paths can be a complex thing, with offers and proposals and time to process on both sides for the whole range of packages and making a pricing scheme.

And it's usual for companies to push out their Dev Boards Revision 1 early, because that's usually what engineers like to play with to make up their mind about a chip, so if you delay that, you're losing potential customers.