This is because Alton Towers have a rule that you are not allowed on any rides as the only person in the vehicle.
This is fine on coasters such as Nemesis which have long trains with people on other rows. It only becomes an issue on the Rapids and Spinball because single riders can’t be paired up with other guests due to social distancing. This means that these two rides can’t accommodate single riders.
This is however, unlikely to be an issue at any other park. Alton Towers is the only park I’ve ever come across this rule of people not being allowed to ride in a vehicle alone. At other parks, including Merlin ones, I’ve witnessed single riders being alone in a ride vehicle, including on comparable rides such as Dragons Fury and other Rapids rides.
In short, it’s an Alton Towers policy which you’re unlikely to encounter elsewhere.