Of course, Jim does not really tell us why he decides that he will never go back to Treasure Island.  So we have to figure it out for ourselves.

First, he got a portion of the treasure that they brought back with them.  We do not know if he used it wisely, but it seems likely that he no longer really needs money.

Second, if you look at the end of the book, he is pretty much saying that what happened to him on the island scared him too much for him to ever want to go back.  I think he has decided that the desire for too much treasure is dangerous and he should be content with what he has.