Errr, I'd guess because there isn't/wasn't many of them? They were only produced for about 3 years (no idea on numbers)back in the 60's, and were then superceded by the more powerful B2 model. In addition, many of the A's were converted to A/B's, which primarily involved changing the C18 engine to a C20, along with a few other amends.
I have never flown an A, but from what I gather if all you want to do is potter about 1 or 2 up, they are ideal.
To say that they not as common as B's is like saying that there's not many Ford Model T's around, so they must have been lousy!