Oh goodie, an excuse to rant about Madoka! You knew I was going to do this
People like this show because they've never watched an actual magical girl show before, or their experience with said shows is limited entirely to Sailor Moon. Magical girls are not as popular as they were in the days of Sailor Moon, and most series that would fit into this genre - of which I would say Madoka is NOT one, any more so than Kill la Kill - fly under the radar. Precure is still going strong in Japan as a merch-selling machine, but in the West at least its very niche, with HeartCatch several years behind us and 90% of the "fans" who came into the show with HeartCatch unwilling to try another season beyond that, solely because its not HeartCatch.
People also like dark and edgy things for some stupid reason I haven't been able to figure out yet. They delight in the suffering of characters because reasons. If its dramatic, who gives a shit whether it's good quality or not. There are also some very big names attached to its production which mean it must be good, because these people are gods of the media and can do no wrong. Because because because.
People also have no idea of what a "deconstruction" actually is, other than that it sounds big and impressive and special and different, and they like to feel that they like different things, and have objectively superior taste in some way. Despite the fact that Madoka is not a deconstruction, playing straight every single trope from the magical girl genre it takes, and just twisting its premise slightly to be grimmer than one would expect from a show of that genre...if you haven't watched many magical girl anime, anyway. For a proper deconstruction of the genre, and one that uses its darker moments to fuel character development - that actually does what fans of Madoka mistakenly claim it does - refer to Princess Tutu. For a mind screw with actual meaning to its flashy imagery - although good luck figuring it out - refer to Revolutionary Girl Utena. For a better example of a shorter magical girl show that plays with convention slightly in its premise but plays its tropes straight in the same fashion, watch Yuki Yuna. Madoka is a very, VERY small, depressingly miserable ripple in a pond far bigger than its fans like to think, and not even a very good example of anything it tries to do.
So, why is Madoka garbage? For one, there is zero character development. The "plot" of the show is driven by shock moments that the characters and the viewer have absolutely no time to process. Yes, these are plot twists you would not necessarily see coming, but this is NOT good storytelling by itself. Any idiot can kill a character off in a show.
A truly skilled writer will take such a scene and show how it affects the characters - how each one reacts, how they cope, and how they move on - IF they move on. This is not something that Urobutcher has EVER done, across any of his shows. Madoka does not put enough time into any of its cast for us to really get a sense of their personalities - relying on the fandom to fill in the gaps, as fandom often does, with idiotic things like shipping - and the ultimate end to the show is an anticlimax. Everything is simply wiped away. That's it.
It's such a bizarre, gutless u-turn that it's no small wonder that fans cried havoc over Rebellion, which was ironically far more in-keeping with the mindless suffering the entire show embodied. It makes no sense. There is no build-up to it, or indication that it will happen, or even a sense of resolution after the fact. It just happens. Like everything else in the show just happens. Why? Because Urobutcher thinks that if a scene is shocking enough it will justify itself. That mindless suffering and slaughter equates to good character development. IT DOES NOT. It is a trigger for development. In Madoka, that never materialises. Not once.
That is ultimately what Madoka is - mindless suffering. Nothing more. Complete and utter garbage. End of story.