Because (I'd guess, as a PhD student in another CS field) any advisor worth its salt would advise a grad student to work on something else first, get tenure, and then maybe approach this problem. Until yesterday, most researchers agreed that the problem was unapproachable. Nowadays being a researcher is a job that requires steady progress, so you must focus on approachable problems. For P vs NP there are tons of results on classes of techniques that _cannot_ work—you'd have to learn those first to make a serious attempt.