Quantum required significant work and changes over the winter period. These weren't planned and came a bit out of the blue. As has been noted, the ride is notably being put back together, but there's still a lot to do. This is different to situations like Black Buccaneer, Ripsaw, etc, where something needed to happen and money wasn't spent and it was just closed.
It's obviously poor that it isn't ready, especially at a time when this is more common within Merlin. If this was uncommon, people would be a lot more forgiving (a bit like when Quantum didn't open until August one year like 12 years ago).
But in fairness, the park are doing something and spending money to get Quantum sorted. It's just taking a long time.