Not sure if they have gotten dimmer. Possibly the opposite: I don't remember having noticed it recently (but I've also travelled a lot less), but I do remember being very annoyed by this many years ago.
No idea why hotels were doing it, neither LEDs nor the electricity for them are that expensive and I doubt anyone wants to be stuck in a dark cave.
Is it just me, or have hotel rooms gotten dimmer?
It seems less cheerful.