Precisely to cash in on the success of the X-Men concept which was riding high at the time.
New Mutants had proved there was a market for more mutant stories, and who better to focus on than the ones who started it all, most of whom were in character limbo. You got instant identifcation with minimum derailing of other plotlines to bring them together.
Restoring Jean Grey was a major event and so was a good launch point. Note I'm not saying that bringing her back was a good idea (because I don't think it was), but I can see the business logic behind it.