Why do lower tooth extractions hurt more than upper?

Not always: Depends on the difficulty of the extraction. Bone is denser on the lower which may explain more discomfort.

That's subjective: Each tooth / situation is different. I don't know that extractions of lower teeth are more painful than uppers. It is possible that you did not get adequately numb for the procedure on the lower. Hope this helps.

Not sure: While it is true that lower posterior teeth are more painful after extraction than uppers, upper anterior teeth may be more painful than lower anteriors. Has to due with characteristics of local innervation. Just like dental injections are more painful in the upper anterior than other oral areas.