Easier to waive flags than learn or ask people to put on tefilin...
It's also easier to flat out, blanketly, denounce a whole group of people whose ideology you disagree with than to recognize what they are doing right.
It just happens to be, that generally speaking, the "flaggers" are amongst the most dedicated to learning and mivtzoim and "the Rebbe's inyonim" in general. Of course there are exceptions, but if you look at the average "flagger" bochur you'll find that to be the case.
I have plenty of problems with them and what they do, but I can also recognize what they do right.