You might ask why can't we just add ring signatures, stealth addresses, and confidential transactions to bitcoin... lots of reasons, but let's start with the fact that it makes the thing unauditable.

Tell me exactly how much monero exists... can you?  Spin up your node and audit it.  That's right, you can't.  

There could be an inflation exploit and it's possible no one even knows about it.

Satoshi spoke on this subject.  His opinion was that adding all this fancy shit was too risky.  I agree with him.  That's why we can't have privacy.  

Privacy is one of those things, it's either baked into the protocol from day 1, or you will never get it.

Ultimately in the case of bitcoin, it was a tradeoff.  No full privacy but full verifiability.