Why couldnt Mrs. Whatsit, Mrs. Who, and Mrs. Which go to Camazotz?

We're not
told exactly why Mrs. Whatsit, Mrs. Which, and Mrs. Who can't go to Camazotz. We know they are
former stars, beings on the side of good and consisting of pure goodness themselves. Camazotz,
on the other hand, is completely covered by the dark Thing, which is evil itself. The
implication is that Mrs. Whatsit, Mrs. Which, and Mrs. Who, being purely good, can't stay on a
purely evil planet, though Mrs. Whatsit does accompany the children there for a short
time.

What Mrs. Whatsit, Mrs. Which, and Mrs. Who can do, however, is give
the three children gifts to lean into as they seek their father on Camazotz. Mrs. Whatsit, for
example, gives Meg her faults, which she suggests will be good weapons. Mrs. Who gives Meg her
spectacles, which Meg discovers come in very handy on Camazotz.