Studies found that about 90% of visitors to Disneyland were from West of the Mississippi. Having something on the East coast and within hours drives of major populations centers was a no brainer. I believe that New Orleans was also in consideration at one point as Walt was very fond of the area.
The story goes that after flying through Florida, he saw where 2 major highways crossed and thought that will be the place. He then sent one of his lawyers to investigate the site and start real estate purchasing. To cover his tracks, he would fly out of Orlando and then through other aiports priot to returning to WD in order to throw off anyone that might be catching on or recognize him.
I'm sure more WDW weenies can chime in at this point with more in depth history. I'll spare you the details and just tell you to check out the Walt Disney biography. Its pretty interesting.
Also, the original airport was actually a small Air Force base that was on the verge of closing. It was called McConnel Air Force Base, hence the modern day airport code of MCO. Back in like 1999, I think, my driver on the way to the airport pointed out a bluish ramshackeled building and said that was all that was left of the original airport and that it was the actual main passenger terminal. It was a decrepit building about the size of your average supermarket. They have since knocked it down and built over it as part of the airport expansion, I am told.