It could be because it's creation Dogecoin was supposed to be a joke, a Memecoin, which was not supposed to be taken seriously by anyone and just have some kicks and laughs at the fact it was an asset with relatively high liquidity on the market.
Also, it could be Dogecoin has pretty much an unlimited supply and hence it is considered to be an inflationary asset, which could be counter-productive in terms of keeping it as a reserve assets within the Federal government of the United States.

It does not matter whether Dogecoin is the favorite decentralized asset of Elon Musk, he also need to keep some façade of common sense and not make to believe the rest of the American population he is literally pushing an asset into the strategy reserve because it is favorite among the thousands of alternative currencies floating around in this space.