It's because in this year, 19 of KnY's volumes are selling better than all 96 of OP's old volumes. That's because the anime released last year resulting in people checking out the short manga and millions buying in bulk. But even if we take OP's most recent volumes in comparison to KnY's most recent volumes, they're still dwarfed, with KnY selling millions more.
Even though OP's sales are declining it's not doing horrendously, it's merely that KnY is doing far, far better.