Can anyone summarise why Corbett is so unpopular? All I know is that the Jerry Sandusky case hurt him somehow, but I'm not sure exactly how and why.
1)
The Sandusky case: Many people think Corbett slow-walked the investigation so that it didn't interfere with his gubernatorial run in 2010 (he was Attorney General, and actually a fairly popular one, from 2005-2011). Kathleen Kane took advantage of this by pledging to run a full investigation into it, which is part of why she won by such a huge margin. This might not seem like a big deal to out of staters, but it's pretty huge in PA.
2)
Education: Corbett cut education funding drastically. He pretty much eviscerated the public school system, and ended up closing dozens of schools. This pissed off Democrats and independents of course, but even many Republicans, some of whom tend to be more moderate in PA than in other parts of the country.
3)
Failed privatization: Corbett tried to privatize both the state lottery system and state liquor system. He failed on both counts, pissing off the left for trying and the right for failing.
4)
Social issues: Corbett is infamous for saying women should "just close their eyes" while getting their mandatory transvaginal ultrasound. He also compared gay marriage to incest, and used taxpayer money to appeal the court decision since Kane did not. Despite throwing his lot in with the Pennsyltuckians on this, he then pissed them off as well by not appealing the decision. Many on the right wing blogs are calling him a RINO squish who "caved to the Sodomites".