Why is this hated so much?

For one thing, it seems to resemble a bad western medieval story with Mulan's story grafted on. Whoever made this film did almost no research into ancient China (very little compared to the team that made the cartoon in the 90s) and just seemed to improvise everything. An actual Chinese woman on YouTube explained it really well, like the unnecessary witch character and how she was portrayed, versus how they really were treated in ancient China. They also got the Chi concept so wrong, I don't think even drawing a map would have helped the idiots who made this movie. They made it seem like a magic power that only men had, and Mulan was the rare super-girl to have it, when Chi is not seen that way at all in China, not even today. The writing was pretty bad too, with a lot of logic fails. It's like the movie-makers forgot what they were doing around the last third of the film, and got lost on the way to the finish line.

Taking away the more fun characters was also a big mistake, such as the snarky grandma, handsome, honorable Shang, the funny goofball soldiers, and Mushu.

Hell, neither this film, nor the cartoon seemed to understand how matchmakers work. They made it seem like the girl had to dress up as a bride, go see the matchmaker, and get married the same day. Doesn't work that way in China, past or present. And for some reason, Mulan's bridal look was really ugly in this film. I've seen ancient Chinese costume and makeup, and for some reason, they chose the ugliest look they could find for Mulan's botched session with the matchmaker.

The point behind Mulan going off to war, disguised as a boy, was to save her dad. She cared about him, as well as seeing it was her duty to keep him safe, which forms the core of Chinese culture: filial piety. This movie seems to forget that, that Mulan was going off to find glory and acceptance instead of putting her life in danger to save her dad from the horrors of war.

It's kind of interesting but these remakes get complaints if they stick too close to the original animation or if they drift away too much. It's a Catch-22.

Nostalgia bias.