Any coaster will experience some form of change during colder weather (5℃ and under ). It's only natural.
In the UK we have such varied weather throughout the year, with minus temperatures common in winter, but temperatures occasionally going into high 20s/early 30s, and this can wreck havock with some coasters. I understand Oblivion can become very temperamental in heat, particularly if we have a sudden change such as a heatwave.
Our climate is something which should be taken into account when a ride is designed (I.E. the point of 'warming up'), but Nemesis is probably one of the most reliable coasters at Alton at any rate. Yes, it may limp around the track a little, but it manages it perfectly fine, and is fully capable of operating.
I've seen Air at February Half Term running a cold craft, to have it very nearly valley over the garage. Fortunately it just scraped around, but it was enough for the ride PLC to shut down the ride, as the craft took so long it assumed it was stuck.
Th13teen equally will struggle when it's cold. If I recall correctly it needs at least half a train to make a complete circuit first thing in the morning, which is why it caused so many issues when trying to run it for ERT.
At Drayton they only have Shockwave and G-Force. However, if I had to guess at one being able to make a successful circuit in the cold I'd guess Shockwave. G-Force can really crawl through the Cuban 8 first thing, so in very low temperatures I wouldn't be one bit surprised if it vallyed.
As I said above though, getting a train all the way around is only one half of the battle. It may well go around, but will it do it fast enough to keep running, or like Air will it shut down?
Then you have the extra staff to get in to run the ride, and with a lot of staff being seasonal it could be difficult to round up a whole team, at a time when people are wanting time off with family and friends. I can't imagine it being cost effective to train new staff for just a few weeks.
Also, as Benzin points out, there is the need to carry out maintenance work on the rides. This will need time to be done thoroughly, as well as outside contractors such as LTCI to perform their inspections. If something comes up here the ride could easily be put out of action for the duration of the event, which wouldn't make a lot of people happy (Both staff and guests).
One more thing to consider could be insurance. The rides will have a safe minimum operating temperature placed upon them, below which the ride can or should not be ran. If they were to operate below this point and an accident happened I expect it could all come down on Drayton for operating outside of their safe conditions, and I wouldn't be surprised if the insurers had some similar policy, which could make it a very costly experience for the park.
I can understand from Drayton's point of view why they'd sooner not, or cannot open one of the coasters during Winter. The event is very heavily targeted towards children, so I imagine the time, money, and effort of opening up one of the coasters would be far from justified for the amount of riders they get. Parents will be their regardless for their children, and a single coaster isn't going to bring the teenagers and adults flooding in. It would be nice to see perhaps Maelstrom added in, if they could, but as above, I doubt it would be financially viable or justified.
It would be nice to see the event grow, but for now at any rate I can't see there being much change.