Because Mamdani is not "far far left" unless you live in the American echo chamber which treats anything not right-wing as a leftist threat [1]. On a global stage, where actual political systems exist beyond America's basic, captured two-party system, the words "left" and "right" have established political meaning.
because Americans are incredibly stupid when it comes to understanding class politics so they just throw words like marxist and communist around without knowing what they mean (source: I am American)
Because in the US feeding kids for free in schools is seen as communism, but that's not what communism is. You can't say this dude is a communist or an islamist because words have a meaning and these meanings transcend American borders
[1] https://www.aclu.org/news/national-security/how-nspm-7-seeks...