My impression is that there's a lot of discretion in an MS diagnosis, that it's a set of symptoms with no other identified cause. I.e., it's like a process of elimination.
I don't know much about MS so I can't make any claim really, but it seems like there might be room for those symptoms to be interpreted very differently in other countries with different training and testing protocols.
Definitely some selection bias in there, but not "the US vs the world".