It will never be on par, it is just the nature of the material. But I think you are being a little selective in your history. Snow White was the highest gossing film so far when it was released; 2 digitally animated films were in the top ten grossing in 2012 and 2011; in 2010 5 of the top ten were digitally animated (and Toy Story 3 in the #1 spot).
But if you wan't something a little more concrete - get rid of the Hayes Code, it was more restrictive on animation because of the idea that it appealed mostly to the youth. Once that was removed, people started expermenting more with 'adult' animation.