ArtemisDane

In all likelihood, it's not the school that's doing the admitting - it's the professor. Schools often have research professors who neither advise students nor teach. They have no direct interaction with anyone outside their lab. So they can't just pluck students from the cohort like other professors. As a result, when they need graduate students they tend to recruit them directly and put them to work immediately as research assistants. This can be both good and bad for the student. If it works out, it tends to be great since you've got a professor invested in your success from day one. However, if you start to have issues, you're locked into that professor more than ordinarily be. However, from the professor's standpoint, they can't wait around until April 15th to potentially get a 'no' because they won't have any time to recruit someone else. It's actually a difficult situation for everyone. The professor doing the recruiting is probably a rock star researcher at the school - they keep them around despite the fact that they don't involve themselves in teaching or departmental politics because they're so good at bringing in the grant money. The applicants they pick are usually the cream of the crop from the applications - which means those students normally have applications (with a good shot of success) at much better schools. The professor needs to know immediately whether they should look elsewhere while the student has to consider whether the potential upside is worth hitching their wagon to this guy they barely know at a school that is (probably) much less prestigious than their other options.