I think one big reason is that regulators don't want the US to miss out on a large wave of value creation. If Bitcoin becomes a mainstream currency, it could be a very good thing for the US. If US citizens own a large amount of the BTC in circulation that could lead to more investment in the US economy and a larger tax base.

Beyond Bitcoin, if crypto tokens begin to supplant public stocks (which I'm not saying they will), it would be incredibly foolish to cut off US citizens from those markets