My guess is that Ryanair don't want a ruling in the high court in any country to go in that direction. Especially not at this time as the European Court of Justice soon will try a similar case out of Belgium.
After the Norwegian Government announced the introduction of an extra tax on air travel Ryanair is leaving Rygge and instead moving some flights to OSL. However, it is expected there will be no crew base in Norway anymore.
A ruling in the highest court would be definite in Norway and might also prove to be a precedent for other court rulings. Hence Ryanair are withdrawing its appeal.