Easyjet are aiming to reduce the proportion of 319's in the fleet, hence leaning towards the larger Airbus model. C Series, though a great propositon, would mean a huge amount of duplication, complexity and ultimately additional costs. No doubt a full evaluation will have been done on that but my guess would be that EasyJet have a great commercial arrangement with Airbus hence, on balance unlikely to be a business case for adding c series to the fleet....