Who knows, but it probably dates back to some arcane calculation that someone struck by a 5kg plane is likely to survive while not with a heavier plane... perhaps related to the "knife-edge wing" rule that the leading radius cannot be smaller than 2mm... and no metal propellers. Or perhaps even to some ancient shipping regulations.

Whatever the reasons, it isn't likely to change anytime soon, although I can see an argument for having glow models meet the weight requirement with fuel. Probably put the final nail in the coffin for glow.

Since it isn't going to change, why don't some of the innovators out there start working from a 1.7 or 1.8-meter standard that would permit all kinds of design flexibility without bumping up against the 5kg limit? Sebart is already 'sort of' doing this with the Mythos.

The one (almost) universal constant is that people would rather complain instead of "working the problem".