I don’t recall why that decision was made, but there could be a couple reasons.

First, it isn’t easy technically. Especially back when you’re designing the very first decentralized cryptocurrency and have no prior experience informing your design. ZCash, Monero, MimbleWimble and others came later after learning from Bitcoin, and there’s zero chance they could have come first.

Second, shielded transactions risk undetected inflation bugs, which actually happened to ZCash some years ago.

Third, Bitcoin was designed shortly after the Liberty Dollar founder was arrested and jailed, and everyone in Bitcoin was concerned about that too, including Satoshi. He may have decided just not to push his luck.