The relationship between Universal and Nickelodeon after the Nick studios left and Jimmy Neutron closed was "strange", and I think they wanted to make sure they didn't paint themselves into a corner by creating something big that couldn't easily be rethemed or removed in case of troubles down the line. Just my read on it anyway. Nick opened a hotel near Disney with their own "attractions," lands in Cedar Fair parks and Mall of America and eventually their own theme park.
For whatever reason Universal seems content having Nick and Nick Jr characters in the parade, around the park, on the bags, and in one dedicated gift shop, but they drew a line on any more attractions a long time ago.