That has had me wondering, too.
I think it may have been a question of having had the chance to close them once the blowout was apparent before the explosion.
I have been wondering why no one has attempted to hook up lines to the existing BOP and close those rams (or maybe they did), but there may be other problems.
Is there still drill pipe in the hole?
Can another stack be bolted onto the existing wellhead if the current BOP was removed? Is the casing in good enough shape to withstand the pressure? I am sure these questions have been asked elsewhere.
I know that a ROV has tried to close the rams.