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.