Originally Posted by
trip
No middle seats, a big win, why on earth did the CSeries go with a middle seat?
Because the DC-9 / MD-80 had a 2-3 seating configuration. Although a clean sheet design, they went with that size fuselage.
It also helps fuel efficiency for that seating, as well as baggage capacity. Think about a 1-1 seating configuration 50 seats long. Think about a 5-5 seating configuration 10 seats long (both for 100 seats). Both extremes are inefficient. There is a sweet spot in between.