Because now it's exactly like every other piece of open source software, so lawyers can stop debating whether it's better or worse than an implicit grant.

Like every other piece of OS software that is known to be covered by patents

Just because software is not known to be covered by patents doesn't mean you don't have to worry about software patents.

It doesn't - but it does make lawyers fret less.

It's also more compatible (which, tbh, is actually more important practically) and that was the main problem with the whole Apache thing.