Maybe because there are, or should be, consequences of making false accusations, including defamation suits and loss of readership.
I'm personally waiting for the UVA chapter of Phi Kappa Psi, and/or national organization, to sue, considering they were accused of "ritualized rape", implying there has been a long string of them, and this ONE victim finally spoke out.
Rolling Stone could stop the damage in 3 simple steps, and probably avoid losing a lawsuit:
1) Make a sincere apology to those they FALSELY ACCUSED of a heinous crime.
2) Fire the author and editor responsible for that article. Even if the author had an agenda, the editor should be there to check that the homework was done, i.e. calling someone to confirm the event happened.
3) Announce a policy of two editors double-checking sources when there are criminal allegations being made in an article.