Well he did to make people wonder what and why the other movies were going to be about.

My opinion is the vision he had for the prequel trilogy wasn't do-able in the 70s and 80s because the technology he was hoping for wasn't available back then. He probably put them off to see if there would be any advancement in computers and what Hollywood would be able to do in the future. If you think back on episodes 4-6 versus 1-3, you can see that 1-3 had more of the interactive, explosive space battles as well as the more in-depth planet battles in addition to Coruscant. Imagine trying to create a city that covers an entire planet with only the technology available in the 70s-80s? I definitely think he had the right idea in waiting to do episodes 1-3 until recent times. But, that's just my opinion.

I think it was because fans wanted to see what Darth Vader's (Anakin's) childhood was like. And some other charector's origins.

He originally did just Star Wars (now Episode IV) and it was a hit so he chose to continue the story, it wasn't until about half way through the full production that he realized that where he had started the story was in the middle and not the beginning, though becca85 is right in that he also realized that in order to do the beginning story with the number of Jedi rising, and the scale and scope of the battles and technology that it would be some time before he could actually do the movies, and even at one point considered not continuing on with them. The final decision to make them came when advances in 3D Modeling technology came about and the knowledge that he could give the movies the full scope that he was looking for.

I agree with Becca85

I read in a book that when Lucas first thought of Star Wars, he thought of Luke having a father. Then, when he was writing the script, Vader evolved to be Luke's father, so he wanted to explain the character's origins. Along with doing that, he also evolved to get the ships. The Jedi Starfighters in episode III show some resemblance to TIE Fighters, and the ARC-170 Starfighters evolve into the X-Wing.

He was originally only going to do Star Wars.
The two sequels and the prequel trilogy were an afterthought.