As mentioned in earlier answers, branding and management (pay, collective bargaining agreements, etc.) are certainly two major factors. And legacy airlines are mindful of the risk to see their profit cannibalised by the new cheap fares so they need to maintain as much segmentation/price discrimination as possible.
Interestingly, the two strategies are not exclusive. Air France/KLM has several low-cost subsidiaries (Transavia, Transavia-France and HOP!) but also introduced many low-costs tactics (no meal, surcharge for hold luggage…) on short-haul European routes on both legacy airlines.
The thing is that some elements of the low-cost model (paid refreshments, quick turnaround, routes to cheap airports or use low-cost terminals at major airports, single-class cabin with as many seats as possible, identical aircrafts…) can only be introduced for the whole airplane or even the whole fleet. And for a highly price-sensitive market segment, merely using some elements of the model to offer lower prices is not enough, you need to go all in to ensure your fares are as low as those of your direct competitors (say Easyjet, not necessarily Ryanair).