The main culprits are programs that were written before the standard existed in addition to many people that just don't know about it.

Who should enforce this standard? I would say community pressure does it pretty well at the moment. A standard does not need to be enforced and the XDG base directory one is still useful. The more awareness exists and the more people want this and think it is useful the better. It also has the advantage that no other competing standard exists as far as I know but it still has some downsides.