Many possibilities. Colossus struggles to make it around the final corner in low temperatures, and in temperatures as low as we're seeing it's got a too high likelihood of it valleying for it to open. Stealth doesn't have this problem, as it only has to launch it's way around a 13 second circuit to the brakes. Of course it still has the risk of rolling back, but this seems to happen less frequently nowadays, and when it does it's considerably easier to recover than Colossus. Other reasons could be that they're different ages and different types of rides, for example maybe Colossus's lift motor has problems in low temperatures just like Saw's does. Stealth again wouldn't have the same problem as they use different systems which might be able to deal with the cold weather.
As for it failing to open in years gone by, we can only really make assumptions. Maybe as Intamin have developed, they've replaced older parts with newer ones more able to withstand colder temperatures? Maybe it just gets better maintenance now than it did before?