Because it isn't very good. Carlin has been known to make things up, regularly takes individual instances to represent whole peoples and eras, exaggerates and makes misleading analogies, and when he makes errors (which is too often) he is reluctant to correct them. He became a regular feature over on /r/badhistory for a reason.

https://www.reddit.com/r/badhistory/comments/6swvvq/opinion_of_this_sub_on_dan_carlins_hardcore/

https://www.reddit.com/r/badhistory/comments/385860/we_all_know_about_godwins_law_and_poes_but_i/

https://www.reddit.com/r/badhistory/comments/3v63nh/dan_carlins_blueprint_for_armageddon_has_7/

Part of the problem is (and it's a fundamental thing), he's not a historian, and says so as a way to shield himself from criticism, rather than use it to explain why he's made mistakes. His fans use the same excuse, and don't take criticism of his work well. Throughout his work he hasn't shown much compunction about improving, either.