From this site
The Seven Deadly Sins of Gilligan’s Island
The theory is quite simple. Each of the seven characters on the island represents each of the seven deadly sins.
Most obvious is the Professor, who fits pride to a T. Any man who can make a radio out of some wire and two coconuts has to be pretty cocky.
For the sin of envy we need look no further than Maryann, who may have worn those skimpy little tops, but could never achieve Ginger’s glamour.
And who could doubt for a moment that Ginger is lust incarnate? Sure, the kids were supposed to think she was acting, but we all know what being deprived episode after episode was doing to her. You know and I know that glazed look wasn’t boredom.
What kind of person takes a trunk full of cash on a three-hour cruise? Mr. Howell obviously represents greed.
We are now left with three characters and three Deadly Sins. We have Gilligan, the Skipper and Mrs. Howell to whom we must match gluttony, sloth and anger. As you can see, there is a Gilligan problem here.
Certainly we can further eliminate Mrs Howell from this equation by connecting her with sloth. She did nothing of use whatever during her many years on the island, and everybody knows it.
This leaves anger and gluttony, either of which the Skipper had no shortage. He was, after all, a big guy with the tendency to hit Gilligan with his hat at least once an episode. After much consideration, I have decided that he can easily do double-duty, covering the two remaining Deadly Sins.
So here we have the Seven Deadly Sins trapped in an endlessly recurring Hell of hope followed by denial and despair, forced to live with each other in our TVs until the last re-run ends. And who is their captor? What keeps them trapped there?
Gilligan.
Gilligan is Satan! Think about it.
To answer your question, Thurston Howell III was lured there, without his knowledge, by Satan.