Universal wanted to move into new franchises and they wanted to cash in on the Simpsons, which is still ongoing as of today. BTTF, as popular as it is, was getting outdated by the mid-2000s and 2015 was approaching near so it made things a bit awkward for Universal if we continue to ride it past 2015. Also, look at the BTTF queue videos for example, you can clearly see in the age of HD TV and the early years of streaming service, how they looked like they were shot in the 1980s.
Unlike Disney, Universal attractions are based on IPs that were and are clearly connected with the general, modern audience rather than children/fantasy rides and touch Disney has. So Universal has to adapt and survive in order to keep the crowds coming in and bringing in more money.