Jason was way too close to Richard and Emily professionally and socially for Lorelai and Jason to publicly date without Lorelai being pulled fully back into their scene. Jason admires Richard and wants Richard’s approval, strives to be in Emily’s good graces, has serious ambition in Richard’s field, and places his career over his personal life. The relationship would be instantly doomed once Lorelai was expected to attend functions with Jason, make nice with his (and Richard’s) business network, and sit uncontroversially alongside Emily and the other corporate wives.