It has urgency, also a plea, also is it's own strap line, also says stones, but there is a hole now I think, it doesn't include monuments like henges etc.
SOS is certainly snappy, but it has a couple of obvious problems...
a) It sort of excludes sites like Thornborough which have no stones.
b) It's such a common acronym that anyone searching for it on the Web would be swamped by other stuff.