simple. Its not cost effective. The one real lesson from DAK is that all that park did really was redistribute the guests they already had. Thus, a fifth park is not cost effective in any shape or form.
I've often seen the 'fact' of guest redistribution versus actual growth in attendance thrown around, and I just want to take a few moments to point out that it is likely incorrect.
In 1988, the estimated combined attendance of both MK and EPCOT was 25.1 million.
In 1992, the three parks at WDW took in estimated attendance of:
Magic Kingdom: 12 million
EPCOT: 10 million
Disney-MGM Studios: 8 million
Although this represents a combined drop between MK and EPCOT of 3 million people, it represents a total increase in attendance of 5 million visitors over the previous 1988 attendance record.
By 2007, attendance was estimated to have reached:
Magic Kingdom: 17 million
Epcot: 10.9 million
DHS: 9.51 million
Animal Kingdom: 9.49 million
Given these industry estimates, attendance at MK between 1988 and 2008 could have grown anywhere between 5 and 40%, depending in separate EPCOT performance in the mid 1980s. EPCOT attendance has steadily grown at about 1% a year over the past 10 years.
Although there has been evidence in the past, and statements to the effect that Disney was not necessarily happy with initial performance or ticket draw at the gates, it is plainly obvious that the two most recent parks draw significant visitor totals, ranking as the 7th and 8th most popular amusement parks worldwide, respectively. And all the while, still allowing an overall increase in attendance at MK and EPCOT.
Total attendance at all gates in 2007 was in excess of 47 million, nearly DOUBLE attendance recorded in 1988, and 35% higher than 1992.
That being said, the economic considerations regarding an additional gate take into consideration countless additional details beyond simply total attendance. Infrastructure issues, permitting, design and imagineering, securing investment partners, ability to merchandise, etc are all significant factors in any expansion plans.
Cheers!
sources:
http://www.ibiblio.org/darlene/coaster/FAQ/FAQ.attendance
http://74.125.45.104/search?q=cache...attendance+estimates&hl=en&ct=clnk&cd=1&gl=us
http://www.connectingindustry.com/downloads/pwteaerasupp.pdf