This is most probably because no-fly list is a pile of steaming crap, they had Senators, Congressmen, military veterans, toddlers, etc. on the list. 99.99% it has nothing to do with this specific person (though of course not being a Senator, but being a Muslim from Qatar doesn't exactly help). It's not targeting, it's the opposite.

I'm not sure though why he couldn't fly to Mexico instead - is DHS no-fly list mandatory for other countries?