I would assume that there is some historical precedent for 650b as a road bike wheel size.
Bike technology doesnt always evolve in ways that make sense when seen from current context.

However the primary reason nowadays is probably marketing perceptions; 26" is for mountain bikes, you'll get looked at wierd on club rides if you have mtb wheels on your road bike.

Same reason 700c mountain bikes are called 29ers.



check out michelin WildRun'r 26x1.1 (559 mtb ertd) comes with pncture resistance