Because either people have short memories or weren't following the RF at the time.
Kate wasn't popular at all. And it started from the very beginning. You had journalists who disliked her because she wouldn't play the media game and kept quiet about her relationship for years. Then you had the people who thought she wasn't good enough for William due to her non aristocratic background (because having parents who are self-made and built their own business is so horrible, right?!) then you had trolls who only saw her as a princess in waiting and was doing nothing but to party and never had a job. The narrative was that her and her mother plotted and were desperate for the crown.
A lot of people seem to think that these attacks stopped right after she got married but it really didn't. The press needs a villain and that villain was Kate. From 2011 to 2016, you had articles about feuds with Beatrice and Eugenie, with the Queen, that her and William were keeping George away from Charles, that they are lazy (I did agree with this at the time), that Kate had an eating disorder, some of the press blaming her for the topless photos scandal and the nurse committing suicide after a prank.
If you look at the comments on Katemiddletonreview.com, which is supposed to be pro Kate blog, 90% of them are negative
Even after Meghan first came on the scene, look back at the articles, documentaries in 2017-2018, she was branded as the anti-Kate, modern, fun, breath of fresh air vs boring, old fashioned and traditional.
Then, unfortunately, Meghan became the new villain and they finally left her alone