They were available to buy in July so must be a recent change.Thorpe Park currently not allowing pass holder pre-book for any Fright Nights dates (days either side of Fright Nights are still available).
This strikes me as practice that is in extremely poor taste, seeing if they can fill the park with on the day guests before offering any tickets to pass holders during their busiest time of year. Access to Fright Nights is also one of the advertised perks of the premium annual pass over the standard pass so I'd even argue that they are operating in a grey area in terms of false advertising (although they will no doubt release an extremely small quantity of annual pass tickets at some stage to counter this).
It’s also sadly something Merlin are doing at other parks, Towers aren’t allowing passholders to prebook for Fireworks yet if you pay for a day ticket you can book.