Simple they didn't follow the Disney pattern and open a small park at first and keep expanding it overtime.Universal just built a park that was ready for the public at the opening i think out of desperation just to beat Disney but the affect was that they didn't have anything new until those trolleys in 2006 and now harry potter in 09.
Look at mgm when it first opened nothing to the park it is today and epcot is really different too and not to mention Magic Kingdom which has been changing for over 30 years .
No no no no no.
That is not the "Disney" method. That is the "Eisner's cost cutting let's see if we can get away with it" method. Starting out small didn't come along until after the Disneyland Paris fiasco (when they overbuilt on hotels not attractions). Theme parks need to be constantly evolving and changing. That part is correct. Building small and working up is not the only way of doing this. In fact, it is the wrong way of doing it. Instead, building large and then continuing to add and modify from there has proven historically to be the better method.
Magic Kingdom was a complete park when it opened in 1971. You could go there for a whole day (or more) from the beginning. Just because it has been changing for 30 years does not indicate that it was built small to begin with.
Same with Epcot. And Disneyland Paris. And Tokyo Disneyland. These were all complete parks since their inception and have had high attendance throughout their existence by constantly adding bits and pieces. IOA made the right decision in making a complete park to begin with. They expected to attract a large number of visitors in their first years and comparatively, they did. Attendance in 2000 (first complete year of operation) was 6 million (compared to USF's 8.1 million) which is actually right where you would expect a companion park to go. Contrast this with the "built it small" parks like Disney's California Adventure which had only 5 million guests in its first year (compared to over 12 million right next door).
IOA then made the mistake of thinking that is all they had to do. Adding one ride in 3 years is a sign of stagnation. IOA's mistake was not in starting out big, it was in thinking that was all there was to it.
Look at the parks that started out small: Disney's California Adventure, Walt Disney Studios Paris, Hong Kong Disneyland... none of them are considered successful right now. Contrast with Tokyo Disneysea which was built around the same time, but in a complete form. It is one of the top visited theme parks in the world. Tokyo Disneysea continues to add attractions each year. Yes, they built the expensive Tower of Terror. But they have also added smaller stuff liking Raging Spirits and nighttime shows.
So no, IOA did not make a mistake in starting out too big. They may have wanted to wait about a year before opening one of their big attractions, but no one can fault them for having everything ready from day one. What they can be faulted for is then doing almost nothing after that.