Elon Musk likes to joke, meme himself, etc. He likes the attention more than anything else.
I suppose, some experimentation is all he can get. It could be a test for his future plans - perhaps, planning on extending his investments beyond BTC into Alts and wanted to see how other people reacted - no bad of reaction here with DOGE
He is simply using the Doge to do some leverage, and see how his simple tweet and shilling will affect the market. And he is really successful doing it as Doge price really went off the roof. However, with his admittance of buying bitcoin, I'm sure there will be no weight for his Doge tweet anymore. People are going to buy bitcoin because he himself have invested $1.5 billion. So Doge will continue to be a meme coin and from here on in, it will be used by a lot of pump and dump groups.