Why would people not work with him? He's proven again and again and again and again and AGAIN that his songs will be hits. Which in his line of work and for the artists that work with him is all that matters.
Because when he steals someone's song and and uses the plagiarized sections in a new song with the artist he's working with, it's the artist's name that gets dragged through the dirt. Happened with Pink, happened with Avril Lavigne, happened with Daughtry, happened with half the people he's worked with, and most of them don't go back to write with him again after that. Most of the artists who he works with have had substantial hits without him: they don't need him to write a number 1 single for them, especially if there's a risk that he stole it from someone else.
Somehow, I don't anticipate that as many of these artists have strong moral compasses to deter them from working with him haha. especially when i'm sure some of those artists he rips off will eventually get to work with him themselves.
Pretty sure most of the people who have accused him of stealing their songs have not worked with him after that, especially because he sues people for "defamation" if they say he plagiarized.