The chafetz chayim in the Mishna Brura claims that in hot countries that one can daven in short pants etc and quotes that "ein lachosh baze" so why are some communties uptight at peole that wear shorts during davening ?

You refer to Mishna Berura 91 13 who doesn't specifically mention short pants. Harav Ovadia Yosef (Shu"tYehavve Daat IV 8) writes that it is permitted to daven in shorts, but he feels that shorts are not respectable enough so that it would be permitted for the chazan to wear shorts. In a country where it was generally accepted to greet the president or other distinguished officials while wearing shorts, it would be permitted for the chazan as well.