Simple: Hollywood embraces IPs, because it's "Hollywood", which is the excuse Murdy uses. The creative team gets to sharpen their originality with the scarezones (barely), while Murdy schmoozes and obsesses on grabbing those IPs, which marketing uses exclusively to help attract guests in the LA area.
In Orlando there is barely any competition wherein adult Halloween entertainment is concerened, as well as the fact that UOR has a long, established yearly history with HHN. The icons fit like a glove there. It's not a reason why USH doesn't have an icon, nor should it be an excuse, but it allows Murdy to get away with his myopic, self-centered view on creating the event. There are very little traditions he holds sacred, aside from the "kiss goodnight" chainsaws. One could argue that the opening scaremonies are a formality derived by operations.
The sad fact is that they don't want to create the illusion of FOMA for any incoming guest because nothing pushes them in that direction. They have the hottest IPs, and it works for making them the most popular west coast haunt event. I disagree, but hey the turnstiles are still clicking so who's to say they will change it?