Bitcoin is the most popular cryptocurrency and therefore it is understandable why Elon Musk writes constantly about Bitcoin, but why Elon Musk constantly writes about Dogecoin is not clear. In my opinion, most likely Elon Musk invested in Bitcoin and Dogecoin and because of that Elon Musk constantly advertises these two cryptocurrencies on his Twitter, the growth of these two cryptocurrencies started, and this is the main thing.
That's not the main thing when Elon Musk knows cryptocurrency, because he has an unusual mission so he applies the writing in his tweets like that so that everyone has a different opinion by seeing his tweets, because he likes Dogecoin just to get a lot of profit and says Bitcoin is only to make the assets last at a good price, because Bitcoin and Dogecoin are both cryptocurrencies that have been around for a very long time even though they are categorized differently.