It might be to do with how jarring it looks. If this attraction was more subtle with it's colours and fitted in with Thorpe's sky blue colour and the rest of Lost City, then maaaaaaaaaybe this would have been alright. But with colours as bright as this and looking so blatantly out-of-place next to the lake and Colossus/Rush/Zodiac, it looks visually revolting.
This is just humiliating for the park, similarly to Love Island Lates last year and this will only be 'popular' because it's free. I guarantee if this was an upcharge attraction, the majority of guests would tell Thorpe to shove BounceZilla up where the sun don't shine.