Why did they wait so long inbetween each star wars production?  

Instead of going with Episode I to start the Series in 1977,George Lucas decided to go with what he considered the most bankable movies first-Eps IV-VI. As I recall reading from somewhere way back,His original vision was for 9 Movies in 3 trilogies,and so the "middle" one got to lead off..with the fanbase psyched up by those,he went back and did the special editions to bring the fx up to date,and having seen the popularity of the "Original" trilogy,got the "First" trilogy greenlit by the studios..I think the wait was due to waiting for the FX to catch up!

Lets face it these movies are great. I would expect a production of that size, cost, and quality to take that much time.