Why do narcissists abandon their partners?

I think the simple truth is; because you are no challenge and he is bored. Let me explain. When a person chooses someone intellectually, for their utility (whatever it may be), without emotional ties, there is nothing to distinguish them from anyone else they use similarly, no particular, preferential tie to bind them. Whatever they feel they can feel equally for anyone who fills the same need. Narcissistic personality types are frequently both charming and easily bored. When there is no challenge, and no true emotional preference, they become bored and charm someone else. Easy to do, because within certain limits of taste they are unlikely to be very particular about who they charm. In theory, if you can stop them becoming bored you can hang on to them for life...the real question is, would you want to?