I don't know all the reasons, but the main things I've been hearing about are the issues that people have with Ilvermorny(it is a copy/paste of Hogwarts, prioritizes White wizards over Native wizards, gets historical facts wrong), that the Wizarding World has a few unaddressed problems(child endangerment, biases against Slytherins and Hufflepuffs), the new Pottermore with its redesigned test and lengthy articles, and the FBAWTFT franchise that just seems to be milking the Harry Potter franchise. I don't necessarily agree with all these, but they're just what I've heard around the Internet.
tl;dr: People don't like changes to the canon and franchise and are looking at the books with adult eyes and finding fault.
tl;dr: People don't like changes to the canon and franchise and are looking at the books with adult eyes and finding fault.