Despite all its problems, WDW is a giant vacuum cleaner in central Florida that sucks up all the tourists. Most of them probably don't even know about IOA; they're not theme park aficionados like us. Universal should have built their resort in Texas where it could have been the only game in town with all the attention. They'll always be overshadowed by the Mouse in central Florida and made a big mistake by building there.
It's a combination of Universal not getting the word out and Disney using tactics like their multi-day pricing system and Magical Express to essentially trap people on their property. That stuff wasn't done to improve the guest experience, it was done to hurt their competition. They've effectively made visiting the other resorts impractical.
I believe IOA is a victim of saturation and not really a victim of poor marketing. Most people who come to Orlando do know what's there to be done, but with so much to do in this area, plus Disney's monopoly on tourists' time, most people will have to bypass something due to time and money restrictions... hence IOA, and that is a pity because IOA is thematically speaking the most impressive park in Central Florida... after DAK.
I think that in addition to the reasons that have been listed, many people don't even know that Universal has a second theme park. Also, amen to Vfire's post. We need all the help we can get here in the Lone Star state. It could easily knock out our Six Flags parks.
Building a park in central Florida to compete with Disney is just folly. Looks like Legoland is about to make the same mistake and build its second US park in Florida. The area simply does not have enough locals to support all these parks, and tourism to Florida is down and will continue to decrease in the future (airfares up, economy down). Build theme parks where there's good weather (not sticky humidity and rain), lots of middle class locals with families, and where you're close to being the only game in town. The man Walt himself built his park in the perfect place, southern California.
Good morning, son
In twenty years from now
Maybe we'll both sit down and have a few beers
And I can tell you 'bout today
And how I picked you up and everything changed
It was pain
Sunny days and rain
I knew you'd feel the same things...