I guess because in the South East (or maybe England) there aren't that many 300 mile long bits of bridleway that only require the odd road crossing but not much more in the way of tarmac?
Also, it's a progression: SDW in a day was a major challenge for 'early' mtbers, then the double was done and then it was done faster.
It's a recognised route, one that people aspire to ride, setting the 'target' at the next level isn't a bad thing.
My experience of the South Downs is you can ride the same route more than once, but it's never the same twice.
 I think it's one of those things you either 'get' or 'don't get' I'm not going to try and justify someones elses choice of route and personal challenge any further.