I'm actually surprised that it takes hours to fill the backup BGs, but it seems to. Now I'm just gonna guess here. Perhaps people who came a little later and missed the regular BGs don't want a backup BG because it's uncertain if they'll get to ride? Maybe people see that the regular BGs are filled and give up? I've read nearly every post on this thread and somewhere people must be speculating about this, but I can't pinpoint that place. Hope this is a teensy bit helpful, @hcortesis!
The reason backups last longer is simply the huge number of people who are there at rope drop. You’re not dealing with an even flow of people. You’ve got a huge flood during the first 15 minutes and then a more steady drip after that.