Why did he have to go looking for someone else? What did I do wrong? Why wasn’t I enough?

You did nothing wrong.  You probably were a good wife. It is an addiction, not a response.  He was looking for the feeling he gets from the pursuit and the guilty pleasure.  He is always looking for another fix, another high, just like a drug addict.  As long as he is in the addiction, no one will be “good enough”.