Because its uncomfortable lol... As your uterus grows its tighter and you want to make your pregnancy as comfy as you can considering all the other crap you have to deal with lol
I think it's a comfort thing. I don't mind clothes that are tight but they can't be too tight. I am 33 weeks. I also don't wear tight clothes at work. I work with sex offenders. It doesnt send the right message and it draws too much attention to me that I don't want.
Tight pants can also lead to yeast infections which pregnant women can be prone to anyway.
I cant stand wearing anything tight im only 5w1d and I feel horrible when I try to wear pants I literally wear my zumba clothes everyday :)