In addition to all the reasons listed above, it must also be mentioned that IOA (I guess started?) suffers from IOA syndrome. It opened as a "complete park". But the issue is, it isn't really that encompassing when compared to Studios or Disney. It has far too many restricted rides. This, combined with the fact that it has changed Posideon (worse) and added The Suess Trolly (one of the park's best) in 10 years means it isn't that different from when it opened (minus the lagoon boats too).
Think of any other park that could remain that stagnant for 10 years and still get any attendance, let alone 5 million.
Universal made the same mistake OLC did with TDS. They figured it would be so good people would just keep coming and it would need upgrades mnay years apart, not every other year like most big parks, or every year like most local parks. Both parks opened to amazing first year attendance, then stagnated.
However unlike TDS, IOA hasn't yet had expansion to bring it out of this funk. I mean, its fun every couple of months...but why continue to frequent a park that hasn't changed?