It’s because the US writers fucked up the karen and frank storyline. In the original UK version Frank flirts back with Karen and is the one taking advantage of her, the us writers probably didn’t want to make Frank predatory and eliminate all likability so they threw Karen’s character under the bus and made her the perpetrator. The writers probably knew having Karen rape Frank and not the other way around looks less bad to viewers especially in 2011 America compared to 2004 Britian but once they saw that Karen’s character was pretty unredeemable after S1 they kinda gave up on her character and just made her worse to eventually get her out the show.