Honestly, I think the reason it gets included so often is that it is short and can usually fill out a page that one of Williams' longer and more substantive poems (e.g., Spring and All) ends on.
Also, it is one of the better examples of a type of visual/minimalist/free verse mid-20th century american poetry that is super approachable and understandable (read some of the Wallace Stevens stuff for the counter examples).
Plus, it is great for having "Wait, is this really a poem? What is a poem??" discussions in English 101 classes.
And, to be fair, if some random person wrote it, it would get almost no attention at all, but Williams really was a quite good poet. And while this little poem doesn't have much to it, what it has is nicely wrought.
Just my $0.02...