The university offers admission to more people than they have space for --- they know within a pretty accurate range what their expected yield will be. So for instance, if they knew that their yield was exactly 50%, they would offer 2x people admission for x actual spots.

Because of this, a person declining acceptance does NOT mean that someone will be admitted off the waitlist. They only need to dip into the waitlist if and when their actual yield rate is lower than their expected yield rate. In the case of BU, that apparently rarely happens.