The simple answer is 1) because physicists don't want to muck with general relativity which has had many successes across sub-galactic scales, so they are trying as hard as possible to jam a particle into the hole that's been observed when extending GR/Newton to galactic scales, 2) MOND is just an "effective theory" that seems to fit observations but doesn't really have a fully fleshed out theory explaining why this change to gravity happens; this "inelegance" rubs math-oriented physicsts the wrong way.

I've mentioned elsewhere here that astrophysicsts tend to prefer DM because they like particles and mathematical elegance, and astronomers are more open to MOND because they like theories with few parameters that make successful predictions (a reductive generalization, but broadly true I think). JWST has validated many MOND predictions, so I hope people open their minds a little more now, but as I said, neither theory is fully satisfactory in the end.

Edit: the author of this post is an astronomer that has been working in this field for a long time and he posted another article that reviews MOND and LCDM with some history:

https://tritonstation.com/2023/01/05/question-of-the-year-an...