The multiple levels of governments and associated regulations is very much preventing it from happening. If any one of them in any of the jurisdictions the train might travel says no (or drags their feet) it doesn't happen.
[1] https://en.wikipedia.org/wiki/National_Minimum_Drinking_Age_...
I suspect it would take something spectacular (like how the interstate system was created) to actually make meaningful progress. And the irony is once it's in place no one could imagine life without it.
And of course a lot of rail work now is inter-European, so you get different country governments and the EU as well.