Why crayons specifically?

Two reasons, one practical and one editorial.

The practical one: a child who is coloring is doing something with their hands and almost nothing with their defenses. Questions come out sideways and easily in that state, and the sheet buys several minutes for the idea to sit in the room without anybody having to perform interest in it.

The editorial one: a crayon has one line weight, no eraser and no room for a footnote. If an idea survives being drawn with one, what is left is the part that was doing the work.