One other thing to add: the announcement article also explicitly lists MobileCoin as the first type of crypto they've added. I have no idea if there are others

One other thing to add: the announcement article also explicitly lists MobileCoin as the first type of crypto they've added. I have no idea if there are others that satisfy the requirements of speed and untraceability, but I guess that's relevant to those asking 'why not x'. Which is to add to the article's conclusion: it might be too early to judge.