Way cheaper for starters to keep it in Fiji, especially with all the infrastructure already there. Also, I'd imagine that hosting it in the USA, even on a movie set, would open it up to gawkers and intruders ... there'd be people flying their drones in from everywhere chasing spoilers.Don't know why they bother traveling outside the US for it theses days, could easily do it on a Hollywood movie set, becoming so vanilla.
The location is not the problem, nor are the cast, nor are the gimmicks. It's Season 44 people - even the biggest fans are most likely getting fatigued to some extent.