Shittiest take on this community by far.

It's an myriad of reasons from what I can tell. Americans are conditioned to think along the status quo lines even if there is certain degree of freedom of thought. The American corporate media carves the political landscape to intentionally but subtly influence folks to pick either only Democrats or Republicans.

Another reason is that, I suppose rugged invidualism won out in the American society for better mobilisation. As you rightly pointed out, there just isn't grassroots activism among American people (not counting civil and lgbt rights which are undoubtedly grassroots activism and successful ones at that). But this isn't what it used to be. Before and in the early 20th century, there have been other third political parties still gaining respectable number of votes, the last one being the Socialist Party led by Eugene Debbs. He won a respectable 1 million votes as a presidential candidate while campaigning from prison during World War I.

Not sure what happened why political grassroots activism that could counter either Democratic and Republican parties died out, but my guess is that the proliferation of mass media in the 20th century may have had a hand to convince people to stick with two parties, as well as heavy emphasis on individualistic values.