"They are not men."
Translation:Loro non sono uomini.
I think it is because "non" belongs to the verb, you can't put another word between those two. Also, because of the typical sentence structure. Like in English, where the structure always has to be in this order: subject-verb-object. Here it is the same: loro - non sono - uomini. "Non sono" together form the verb part of the sentence.
This is mostly correct. In a nuanced way, which is not common though sometimes useful, saying "non loro sono gli uomini" is centering the negation on "they." It's like being in a crowded room of many groups of people. You are trying to tell the cops who attacked you. The cops points to the wrong group, and you say, (THEY are not the men, but THEY are) "non LORO sono gli uomini, ma LORO," pointing to a different group.
It's all in what you intend to say.