Same reason why JSON prohibits comments: When you parse it, there is no place to represent the comments in the JavaScript object and array tree, so they must be thrown away, therefore it's impossible to write it out with the same comments in the same places.

Same thing with white space: JSON can not round trip white space or comments the way XML or HTML can, because XML and HTML DOM represents comments and white space explicitly.

Also you can't put comments inside elements like <div <--! comment --> class="foo">. There is no way to represent a comment inside an element in XML or HTML DOM.

But CSS lets you, so you can't round trip comments in CSS.