LCCs as a rule will try to stick with one type as much as they can. Commonality is (almost) everything to cost savings. Pilots, maintenance, training, spare parts -- they must leverage the economies of scale.
But for most LCCs, the CS100 & CS300 cannot meet all of their needs on all of their routes. As an LCC, you'll want to fly 180 seats on some routes. So buying the CSeries means they must fly multiple types and give up on commonality and the savings that come with standardization. Unless the price of oil spikes back up to $150/barrel, that's not going to make economic sense.
The big exception would be brand new LCCs (startups) with no planes yet, planning to fly exclusively to smaller airports (city airports, vacation destinations, etc.)
But how many startup LCCs fitting this model will be introduced in the next, say, 5-10 years? Just a handful.
As a concrete example look at FlyMojo, the Malaysian-backed startup LCC that's signed an LOI to buy up to 40 CS100s. With crude at $50 and 4 Ringgit / USD, that venture is going nowhere.
Republic's parent company also hoped to launch a startup US domestic LCC with the CSeries, but again they couldn't make the numbers work.
Plus a lot of existing LCCs (like most airlines) have now locked in oil prices below $50 via futures contracts through 2023, so there's little reason for them to consider introducing a new type now, even if it's more efficient.