The reason "they" might give, as Yellen, Gensler et al have said often in the past, is that they don't want to enable or improve prospects for terrorist financing. The real reason they don't want it, IMO, is because of the risk that it will start being used for transactions as it gains wider acceptance after explicit Gov approval (right now Gov approval is only lightly implied).  It isn't used for transactions much now but the pipes are in place for that to happen.  If people save and, more importantly transact in BTC or ETH, they will have effectively skirted around the real dollar siphon that is attached to the USD savings via monetary policy.  Monetary and fiscal authorities skimmed 30% off our savings in the last few years to fund every crazy idea that came down the pipe (we see it as reduced purchasing power).  That's a lot of power that they aren't going to want to risk giving up....even if only a small fraction of people realize investing and transacting in other than USD is a way of around having their savings funneled off by monetary and fiscal policy.