Escape was a huge mistake, but there still wasn't a property that people see and immediately have to plan a trip for. Even today, Disney has an overwhelming stranglehold on such properties--close your eyes and pick their listed properties out of a bag and you'd likely land on one. Potter and Nintendo are the only two of that status for Universal, although that'll to be all they need. At the time of IOA's conception, they were (respectively) in utero and too risky.
Now, I've always thought the view of Universal being "a glorified Six Flags" before Potter was outrageously unfair, but it existed (and still does, I guess).