You will get a lot of different answers to that question but personally one of the reasons I dislike SGU is that it literally lifts what is clearly a joke from season 10 of SG-1 but runs with it 110% seriously.

I'm not sure what the spoiler tags are for this site so I'm going to try and remain generally vague about what happens while also making a large chunk of text to make it difficult for things to stand out:

At some point in universe a tv show starts up based on the military program. It's classic 90s sci-fi, as in horribly cheesy with extremely cheap effects parodying both the actual concept of SG-1 as well as 90s sci-fi. The US Gov let it run despite how on the nose about the actual program it is because it offers some deniability about the Stargate. Later on there's an episode dealing with a movie being made that pokes a lot of fun at various sci-fi shows from the time, in particular Farscape because the movie being discussed is getting made after "dvd sales did well" regardless of how the original parody show got cancelled. When discussing how the movie should be made someone makes a comment about how the studio execs want to reboot the show but with a "Young Adult" cast and make it into an interpersonal drama show. In other words a CW show. That's what SGU ended up being. The main cast are all early-mid 20s causing drama between each other because the Chad soldier, who literally first appears on screen in SGU fucking another soldier in a supply closet, ends up as the eye candy for the lead female who is the object of affection for the isekai'd nerd who gets whisked away from his perma-online MMO lifestyle to have adventures IN SPAAACE!

As others mention it's very like the BSG remake of the time and in addition to the issue I mention above the whole thing just doesn't feel like a Stargate show at all. It could easily be a random sci-fi show with nothing to do with Stargate which begs the question of why bother to make it when it turned out like that?

SG: Atlantis is similar enough to SG-1, for better or worse, that it still feels like a Stargate show with both the general story and cast fitting the SG-1 templates. While this touches on a lack of originality as a criticism point it does mean it retains enough to be marketable to the existing SG-1 audience.

Plus the characters from both SG-1 and SGA are for the most part extremely memorable and welcome. SG-1 has better core characters and fewer supporting ones but they are all great. General Hammond and Doctor Fraser for example while not being active SG team members are still very much part of the show. SGA however doesn't really gel as a team until Jason Momoa joins the show in season 2 because Ford never gets developed into anything interesting character wise. SGA does have a much larger supporting roster however and almost all are characters the audience enjoy such as Beckett and Zelenka, even the antagonistic ones.