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.

It's very much pop history. He's really not an academic and somewhat hides behind it.

A lot of the community are outright hostile about him to be honest. Largely because he likes getting into sweeping conversations about culture. The intro to his series on the Pacific Theater spends probably 20-30 minutes on his thoughts for why Japanese society was so fanatical and he uses some less than polite language to describe it.

Many online history communities tend to shun him because it's the 'internet' thing to do. My experience with the history department at my university was that anyone was happy when pop-history gets flipped into genuine academic interest. So if Carlin and his narrative style gets you curious about things, I'd say keep listening.

Personally, I like his work but you have to see it for what it is. Hes a “fan of history, tslking about history”. He’s basically us. Not an academic source. Just a guy who retells stories he finds interesting and wants to talk about.

I think he does a good job of clarifying “this is my feeling about” vs “this is what the community says is a fact”

But if you are viewing him as citable source, I’d be wary there.

Best believe I’ll continue to subscribe to his stuff, but, for example, I would never cite “well Dan Carlin said it went like this” as a counter in a debate.