Because it's a bullshit concept. Neither the author, nor the most active users of this term can provide an exhaustive definition of what the fuck it's supposed to mean. Somebody called you with a word which doesn't sound pleasant? Microaggression! Somebody made a comment about your appearance, your skills, your habits, which, if you stretch far enough, can be considered moderately offensive? Microaggression! You teacher reminded you that you assignment must be submitted by Monday? Microaggression! Somebody made an inappropriate joke? Microaggression! Basically, you can call “microaggression” anything you wouldn't like to hear, which makes the said concept utterly ridiculous and absolutely useless.