Why does meg hug mrs whatsit when she appears on ixchel

The three ladies enter the hall and the beasts bow in reverence. Meg wants to hug them, but then realizes that they haven’t completely materialized and are only like beams of light.

Meg can't hug Mrs. Whatsit because none of the three Mrs’s have fully materialized, they are just beams of light. They do this when they don't have the energy to tesser their whole selves to another planet and just tesser their essence to this other planet. They say that they need to leave in a hurry to get to Meg right when she calls them, so they couldn't materialize completely.