What was Ken Kesey's main idean and purpose for writing One Flew Over the Cuckoo's Nest?

Answer 1

Some of the themes are:
1. Conformity As a Threat to Freedom
2. The Contradiction Between Tyranny and Sanity

see the first link, please for much more info about these themes.

As for his purpose, well, Kesey was one of the most vocal advocates of individual freedom versus conformity and authoritarian control (Nurse Ratched)
So, the novel exemplifies this attitude. McMurphy McMurphy represents sexuality, freedom, and self-determination—characteristics that clash with the oppressed ward, which is controlled by Nurse Ratched.

"It was during this time at Stanford that Kesey volunteered to take part in a government-sponsored program analyzing the effects of hallucinogens such as LSD or mescaline on the human mind (allegedly, the CIA was trying to find a weapon to use in the Cold War that was capable of exerting mass mind-control 4 ). This experience changed him dramatically, and he continued using LSD, which he viewed as perception-altering as opposed to hallucinogenic, thus enabling him to reach a higher level of consciousness, for a long time after. His fascination for altered consciousness caused him to accept a job at a local mental hospital. It was here that he gained the inspiration for the various characters of his aforementioned, critically acclaimed novel.
Rumor has it that his having hallucinations about a giant Indian sweeping the floors led to the conception of Chief “Broom” Bromden, though this turned out to be only part of the truth"

So in the book he's making a social commentary based on personal experience. As a fan of LSD/Acid, he was able to view the world with a different perspective, and though he knew that sometimes what he saw wasn't real, it was real to him. His visual experiences affected his thoughts and notions and created who he was currently.

Answer 2

His idea was to show conditions inside a mental hospital and to show his character Mc Murty and what happens to him when he falls under control of "the system" - a common theme of books in the 60's.
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They're, Their, There - Three Different Words.

Careful or you may wind up in my next novel.

Pax - C