I guess part of it is that he didn’t leave much of an impression in the first Avengers, and then Age of Ultron where he did shine is…not great (which don’t get me wrong, I do like the movie, it just leaves a lot to be desired). And then in Civil War the damage has more or less been done by then.
Don’t get me wrong, I like MCU Hawkeye. I like him a lot even. It’s just that he feels kind of superfluous.