Excluding the UK hints that this is, at least in part, politically motivated. In terms of risk the UK is no different to other European countries. We are certainly not immune to coronavirus, and there are currently no restrictions on travel between the UK and the continent.
Even if they don't restrict it to British passport holders travelling to the USA, it's still arguably easier to screen for the virus through one departure point than, say, 30.
There are many more flights terminating in the UK than any other European country. Plus, American students studying abroad in the UK have yet to return home.
