Various federal organizations get involved in single-state issues in their purview all the time, including the CDC. A single state outbreak can quickly become a multi-state outbtreak, then a nationwide outbreak. Measles, being one of the most contagious diseases in existence, is no joke, so even a small outbreak can be cause for serious concern. Measles has a r0 between 12 and 18, covid has an r0 between 1.4 and 2.4; when extrapolated exponentially, things get gnarly at an alarming rate.