Again, it wasn't just that he declared bankruptcy. It's that his business partners forced him out during that bankruptcy. Would you force a CEO out if you thought he was doing a good job?
As for the billionaire thing... people are starting to doubt that. In the words of Mark Cuban (an actual billionaire) "You have to have the billion part in your bank account, not just the air." - http://money.cnn.com/2016/07/21/news/mark-cuban-donald-trump/
In fact, as many have suspected, he thinks Trump may never have even intended to win!
quite possibly true.