This whole issue is one of those things which doesn't make sense. Here's a segment of the 'Empire Strikes Back - Official Collectors Edition' magazine:
pg.40 'The Script'
Lucas recalls: "The story treatment was very, very detailed. It was the whole thng laid out in scene-by-scene order - what happened and practically everything that was said."
This first draft was produced by veteran scripter Leigh Brackett, who had written the screenplays for Rio Bravo and The Big Sleep. The Empire Strikes Back was her last assignment before her untimely death. This left Lucas in a difficult possition:
"After Leigh died, I did a draft in between before we were able to hire another writer. I was faced with going into production and you just can't come up with somebody just on the spur of the moment who would be right."
Later in the writing process Larry Kasdan wroked with Lucas on producing the final screenplay.
George Lucas admits to not liking writing at all. He says: "I'm not a natural writer. I have a routine. I get up and start writing at eight o'clock in the morning and I quit at five. I sit at my desk relentlessly and that's the only way I can do it because I hate it.
So writing doesn't come easy, yet there was this magical story treatment which surfaced out of somewhere ("practically everything that was said") which Brackett used to create the script.
Yet years later 'Making of ESB'
pg.43
Lucas very quickly hammered out a second draft, finishing on April 1, 1978. *omit* "I found it much easier than I'd expected, almost enjoyable,"
back to this ESB mag, Lucas continues:
"The whole thing is like a big doodle. It started over here and then it just goes off to here, then I go off over there, just wherever it leads me - which is the fun part."
Also amusingly: Cinefantastique Fall 1979
Producer George Lucas' original story was adapted into an intricate screenplay by science fantasy writer Leigh Brackett *omit*, but she died in 1978, after completing only a frist draft. Fox announced that "a young, new talent," Larry Kasdan, "revised and completed the screenplay." We can only hope that not too much "revising" was done to the script of a highly respected screenwriter such as Leigh Brackett.