This may sound like a stupid answer, but I believe it is simply down to rollbacks being a rarer occurrence.
In the past, rollbacks would happen semi-regularly. They were almost an expected part of an operational fortnight or something, for example. It was just 'one of those things'; a routine thing.
Following work and upgrades to the ride's system, a Stealth rollback is, by and large, a much rarer occurrence. Yes, it still happens, and it's not a *major* concern. But it's rarer. So that usually means the park's engineering team can identify why it's happened and rectify it. In the long run, it means the ride is open for longer as a result.
On top of that, following the Smiler incident, Merlin have internally made reopening an even more rigorous process in recent years. Those extra checks probably add time to it too.