I think it's the train wreck rubber-necking factor.  I recall when this heiferette was a human teenager and she was getting lots of sympathy from the media and fans mainly because her mom was a raging stage-cow but also because people assumed the character she played on Modern Family reflected her actual personality.  Once the court awarded her emancipation from her mom, though, she ironically became a  mini-me of her mother.  It's weirdly fascinating.  Oh yeah -- and there's also the pedo-creep factor, at least in the beginning, when she was this jail-bait young thing with big tits, but that quickly faded once she hit 18 and now is 20 going on 30-something rode-hard beetus barge.