Not sure I agree. I've lived all over and in numerous extremes. You can dress for the cold, you cannot avoid heat and humidity. I find it easier to ride in 0f in MN than 105f/95% in AL. That said, I won't ride more than about a mile if it's below -10 or -15f.

I agree though that southern states have many more mild days per year than northern states.

Either way, most states should be able to do 15-20% modal share bicycling if they develop better Dutch style facilities.