Everyone has made some good points.

I could very well see that Wickham doesn't like solitude, he needed some extra cash and Lydia had some, Lydia was eager to get married, and if Wickham kinda suggested it, she would pounce at the opportunity, Wickham knew Lydia's parents not to seem overly strict and might not pursue him (yes, he had met Lydia's uncle and aunt - the Gardiners - who might help find him, but they had been pre-disposed to like him as well.)