I'm not sure if any of it might come down to the fact that the SSJ comes in one shape and size..if you look at the competition you can order the 170, 175, 190 or 195 from Embraer, the CRJ700, 900, 1000, CS100/300 or Dash 8 from BBD, ATR 42 and 72 from France etc. Airlines like having regional aircraft that come in slightly different sizes, they can suit them to their own markets, and many airlines happily operate several members of the same family..i wonder if Sukhoi did a stretch would it help? It would eat into the CS300 market, aswell as the badly selling A319Neo/737 Max 7
Could also be because the SSJ's can be delivered a lot quicker than a fleet of E-jets could