It's not so much that Ryanair doesn't care, they just have different priorities. If they delayed everything with the sheer size of the operation they have, not just at STN but all the away based aircraft that fly into STN from the outstations during the first wave, it would have a catastrophic impact across the entire network. You'd have crew eventually going out of hours all over the place towards the end of the day, meaning those evening flights would have to be cancelled anyway to avoid aircraft and crew being displaced for the next day's operation. That would impact far more customers than the unfortunate ones that missed flights at STN. Add to that, if Ryanair are having to cancel evening flights (or indeed into the next day) due to aircraft and crew shortages as a result of accumulated delay, suddenly that becomes much more of a direct airline issue as far as compensation goes. Fundamentally the decisions the airlines make in these situations is down to EU261 payouts. For as long as the regulation doesn't change, the behaviour of airlines in these scenarios won't either.
As for Jet2, yes they have a package holiday business to supply which plays a large part, but additionally the other 12 UK bases to all intents and purposes are unaffected. Plus, given the 15 or so aircraft they had due out of STN that morning were all based aircraft due back at STN later that day, it doesn't have nearly the same widespread, knock-on impact across the network from purely an operational aspect.