Somewhat because of popular YouTube channels that criticized it in the past (like RedLetterMedia and I Hate Everything).
Some of the criticism fell on the acting. People said the characters were very awkward (but the people were meant to be awkward in some of the cases). People said the dialogue was too formal, and some of it is, but keep in mind that a lot of the scenes are based around politics and council discussion. People in those places are very formal.
A lot of people say the politics is what ruined it, which doesn’t make much sense. The prequels were set in a time of peace. They were what led to the start of the Galactic Empire and civil war in the OT. So politics is definitely going to have something to do with the story; so I don’t get why people hate the politics so much.
I could go further, but……