There must always be a Lich King, and the those that hold the leash of Republicans have decided that AOC (and less specifically, any of our less-than-white D representatives) are the new 'face of the Democrats', and are attempting to use her/them as the evil that the populists must get support to fight against. They've done this for decades, it's nothing new, and it'll never end until the Republican party doesn't exist anymore.
They have no policies worth a shit to raise themselves up, so they must attempt to tear down their opponent instead.
As for why her specifically? Because she's intelligent, outspoken, not white, and female. Basically everything that the Republican base hates nowadays, since the party has abandoned any form of actual governance.