They're a business, they go with what will make them sell, regardless of how recently they might have appeared. December's cover was actually quite a while ago though in all fairness, and she's a new face for Harper's, on any edition.
They need, and more importantly want the money at the end of the day. Big star, with a big movie coming out, will bring in more money than some new fresh face.
Lancome are also a big advertiser and she's their new star. Never underestimate the power of advertising when it comes to such decisions.