KnY is still selling due to its amazing anime and the fact that it has a relatively low number of volumes, people are picking up from where the anime left off.
In OP’s case, it’s last volume was volume 95 in December, those who want to buy it already did, and not many people will start reading the entire series now, unless something extraordinary happened. But it’ll sell another 2 million copies when vol 96 comes out in April, you’ll see it on the rankings then.