Why not? Do they have to sign some confidentiality agreement?
The most likely reason is their desire to keep their acceptance quiet until they're actually prepared for the press that the announcement would receive.
My experience with other startup folks I know is that the accelerator is not designed to be a launchpad for the company's own PR purposes. Show some restraint, built your product, gain some organic and home-grown traction and then at, or near, the conclusion of the accelerator, launch your product (demo day, for example). It is to be about the product/service, not the brand of the accelerator that earns street cred.