I believe Satoshi is a privacy expert.
He created an online persona and there is literally nobody else on this planet that knows this persona is linked with the real person behind Satoshi.

Now, let's speculate, just for fun.

1. Intentional choice:
Perhaps Satoshi wanted to increase our need for self-privacy. Perhaps Satoshi knew that privacy can be achieved with Bitcoin, but that it requires a little more effort.

2. Coincidence:

On April 23, 2011, Satoshi said "I have moved on to other things".

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Satoshi Nakamoto <satoshin@gmx.com> Sat, Apr 23, 2011 at 3:40 PM
To: Mike Hearn <mike@plan99.net>
I had a few other things on my mind (as always). One is, are you planning on rejoining the community at some point (eg for code reviews), or is your plan to permanently step back from the limelight?

I've moved on to other things. It's in good hands with Gavin and everyone.

I do hope your BitcoinJ continues to be developed into an alternative client. It gives Java devs something to work on, and it's easier with a simpler foundation that doesn't have to do everything. It'll get critical mass when impatient new users can get started using it while the other one is still downloading the block chain.

Perhaps Satoshi decided to add Ring Signatures and Stealth Addresses to Bitcoin when it was already too late. And perhaps Satoshi Nakamoto became Nicolas van Saberhagen and developed CryptoNote which is the base layer of Monero.

Conclusion:
Unfortunately we can only speculate. But, in my opinion, Satoshi knew about Ring Signatures and Stealth Addresses and the reason he didn't add them to Bitcoin was because he knew that people would speculate that Bitcoin is only built to support illicit activities. Some people still believe it today. The reason I am saying that, is that there is a message from Satoshi to Gavin Andresen that says:

Quote

I wish you wouldn’t keep talking about me as a mysterious shadowy figure, the press just turns that into a pirate currency angle. Maybe instead make it about the open source project and give more credit to your dev contributors; it helps motivate them.

It is obvious that Satoshi Nakamoto wanted to develop a transparent way to transact P2P and that would become widely adopted. Perhaps he intentionally added pseudonymity and not untreacability, so that people wouldn't think Bitcoin was only used for illegal activities. But...