Zcash's anonymous transactions are much more expensive CPU-wise to verify and aren't pruneable, and the cryptography behind it has been much less reviewed (Bitcoin operates on a bunch of very boring standard already established and long-trusted algorithms in comparison!), so I'd be surprised if an established project like Bitcoin adopted them before they were proven in practice. There's a lot of money tied up in Bitcoin, so the project is going to be pretty conservative in how it chooses to change.
Oh, I guess I mixed up some posts with one lamenting that Zcash wasn't incorporated into Bitcoin.
Stealth addresses seem like they give much weaker anonymity guarantees than Zcash, unless you only ever send and receive funds through stealth addresses with others who follow the same precautions.
