Not sure if migrants carried the disease but every source of information (CDC etc) says it is the lack on kids being vaccinated that spreads it.
“
In a given year, more measles cases can occur for any of the following reasons:
an increase in the number of travelers who get measles abroad and bring it into the U.S., and/or
further spread of measles in U.S. communities with pockets of unvaccinated people.”
https://www.cdc.gov/measles/cases-outbreaks.html
The reason the low numbers are concerning is that measles spreads rapidly to unvaccinated kids and the USA has a lot more than it should. They are concerned about the numbers increasing.