The difference is that Longleat don't market their Festival of Light event as a short break, their main target market is day visitors. I can see why Alton Towers might choose to restrict the Santa experience to hotel guests to sell a 'package' experience and make use of the otherwise empty hotel space. There's nothing wrong with this business model, however if they are selling a premium hotel package there need to be enough experiences to make it feel premium and without a proper festive event in the park akin to Longleat's lantern trail I don't think that's the case. For day guests, there is almost nothing to incentivise them at all.