I don't know what makes Elon interested in dogecoin but I think because of the history or maybe because of his friends introduce doge but I'm sure he already invested a lot of money in dogecoin because he always tweeted it, trying to manipulating dogecoin price.
Elon Musk likes Dogecoin just because it can manipulate the price, nothing more, because Dogecoin doesn't have a use case like other coins so there isn't much that Elon Musk can rely on in this case because the first thing he can do is manipulate the price, that is.