Proper LED UV lighting fixtures are not cheap, the problem is Merlin picked really cheap versions. The cheap versions emit a lot of visible purple light alongside the UV, which has a tendency to light everything up in visible light, along side the UV.
Proper LED UV fixtures emit almost no visible light, therefor act like a true backlight. The technology is still relatively new though, so they are very expensive. I would not be surprised for this reason there is less focus on UV scenes in the new attraction.