I think Indiana is in too many marketing areas for other parks to get a large park of it's own (Great America in Northwestern Indiana, Cedar Point to the northeast, Kings Island to the southeast and Six Flags Kentucky Kingdom to the south). With Six Flags and Cedar Fair not interested in building, only buying, that leaves Disney, Universal, Busch, and they tend to concentrate their facilities in high volume tourist areas with year round operation.
I wish I could recall *where* I read it, but I did read that the US market is basically saturated in terms of major theme parks, which is why US theme park operators are expanding overseas.