George didnt run away after chapter 5 because he knew that curly would not stop looking for them ever. Instead of being accused of ra*e it was murder. A bigger crime to most. Also George may have realized that he could never have the dream he wanted, of his own farm, if he was always on the run because of lennie. Especially since you cant take up a farm and run away.
I hope this helps, I read the book about a year ago, so my mind isnt the freshest