If you read the actual Zerocoin paper, you realize how many fundamental changes it requires to the Bitcoin protocol/process. It's not a layer on top of Bitcoin so much as a fundamental rethinking about BTC semantics. And it only works when a large swath of miners actually use it (you can't just use Zerocoin on your own account and expect it to protect you). This will be a long, slow process.

http://spar.isi.jhu.edu/~mgreen/ZerocoinOakland.pdf