Because it's something like win-win-win for them?
1. They don't stifle a new technology for no reason.
2. They can always regulate it later, with the added bonus that they'll be able to stick a bunch of people they want to investigate with non-compliance charges for failing to register at the appropriate time. Similarly, how many people do you think are doing their taxes right on BTC? Easy arbitrary audit material if you can ever link their accounts.
3. I'm not sure the government is opposed to digital money that leaves an immutable, public record of cryptographic witnesses to transactions. The pseudo-anonymity isn't worth much against a pervasive global adversary watching all the pipes. They link the BTC to your actions, the keys to your machine, and you may as well have signed a confession.
So there's little incentive to act quickly and lots of incentive to give the radicals into BTC a chance to slip up on OPSEC.