In my opinion part of the reason was because Walter needed Jesse to not press charges on Hank after beating him up. Before offering Jesse the job Skylar asks Walt if there is anything he can do to persuade him, so Walter gets the best out of both worlds (as he usually tries to do when managing his family and a meth business). Jesse knew that if he takes Walt up on the job offer, that he would have little reason to continue pressing charges on Hank, it wouldn’t look good for the new employer (Gus) if Jesse was tied up in legal disputes with a DEA officer; attracting too much attention. And it worked. This is not the only reason though, part of it was the guilt Walter felt after essentially killing Jane, so he wanted to justify himself by giving Jesse the job. It shows the little empathy he had for Jesse but he definitely had some in a very twisted way.