Because the same tactic wouldn't work again. And this time, he has Lucius Malfoy to pin it on.

Waitaminute...how exactly have we gotten this far without considering the possibility that Lucius did this? It seems tailored to the protections on Hermione more closely than he would usually be able to manage, but he'd know the Hogwarts wards as well as anyone who wasn't Headmaster, and the "eating the feet first" bit could be coincidence. And plausible deniability plus brute force seems exactly like Lucius' style.