They did this to originally award distinctions to conservative economists like Milton Friedman, though today they’re a little bit sneakier and will every now and then sneak an award to someone like Joseph Stieglitz.
You can read more about it in the somewhat sanitized Wikipedia article which only hints at the dispute that lies beneath: https://en.wikipedia.org/wiki/Nobel_Memorial_Prize_in_Econom...
EDIT: I understand the downvotes for the incorrect factual background. I want my main point to be the issue of economics not doing a great job over the last 30-40 years when these awards were begun / the general retreat of Keynesian economics in favor of Neoliberalism. That being said it’s sometimes mentioned that the peak of a scientist’s productivity is right before he or she gets the Nobel (Memorial) Prize due to the resulting fame, so perhaps other economists are to blame.