Rehashing (pun intended) my generic answer to these questions...

There were SHA-2 ASICs way, way before the first Bitcoin mining ASICs. They were not suitable for mining due to I/O limitations: you'd have to push a relatively big load of data for each single hash, and the chips were not fast enough for that. What mining ASICs do is generate most of the data on the fly (by incrementing the nonce) so there is no I/O bottleneck, in addition to the actual hashing. However, the nonce business is easy, so if you can implement the hash, making a miner is only a matter of time and financial sense.