Insider trading laws are formulated around prosecuting persons that have a contractual duty to the company that act on company-internal information before its published to stockholders. Politicians do have unpublished information about laws and regulations that will affect companies in the future, but they don't have any specific duty to the company, and the information is not exactly company-internal, so it doesn't count as insider trading by technical definition.

I've ever seen a proposed reformulation of these laws that would redraw the lines clearly enough to prevent abuse by politicians but not impede legitimate cases. Maybe the best solution is to ban politicians from trading individual stocks, only broad market indexes / mutual funds.