Because of the natural comparison to the MC universe. In their own way they are both taking lead with comic movies but both worlds are compared within cinema and because of how established and polished as a film series the MCU is, the DCEU will always be seen as second best, thus hating upon it in comparison. In their own rights each DC film isn't bad at all, but as we all know these films have to culminate somewhere and lead towards something bigger, it's when they become intertwined where comparisons are drawn to the Marvel's superhero team offering.
Lets make no mistake, the MCU is far more successful at this stage but it too has its flaws. However, it feels more professional and solid overall. You have 10 years of MCU to catch up on so with that level of establishment and now it's run into the finale of Phase 3, it will be drawn upon compared to DC's far fewer offerings and straight into a Justice League movie because, hey, lets bring the team together and draw in some cash.
I think we can all agree that DC are trying to 'keep up' or 'rush' to establish itself in the same merit as MCU. I don't think their films are bad, I just think they've got it wrong in terms of build and establishing what they need to. What is the hurry? Why not make a couple of MoS movies first before meeting The Bat? That in itself was a bit of an anti-climax for me personally. Every body has always wanted to see Supes meet Bats on the big screen and when it finally happened, yes it was good, but not as good as it should have been in my opinion. DC are quickly pushing their movies towards the team collaboration, and only then are they going to produce us with the individual films afterwards rather than before. Do we care that much about seeing Aquaman have a solo movie AFTER Justice League? Some will argue yes as it draws on his backstory, and that's the point I guess, that's only the route DC can go now, but I would have much rather seen each of these individual heroes built up first. That's just my preference and I think it adds more excitement to a team movie.
I don't think people necessarily 'hate' the DCEU, I just think they prefer the MCU more.
Of course, we are born in an era where we are lucky enough to be able enjoy both and that's what I do. I'll always have my preferences but by no means is the DCEU rubbish.
