Kinda curious about whether the poll in this thread is all that accurate... most replied they'll be getting the 4.7... but every 2 out of 3 post seems to indicate most are going with the 5.5... and judging from the backorder on 6 Plus... but can still get the 6 on 9/19 (not all versions - some are 7-10 days)... So either the supply of the Plus is severely limited (like they did last year on the gold 5S)... or the demand (and I guess globally, the Plus is likely in MUCH higher demand) for the Plus is way bigger than even Apple expected....
I'm 100% sure Apple did not make enough of the Plus variant. I don't think they expected this kind of demand for the bigger iPhone 6. Many people have preordered the iPhone 6 4.7 as well but it's still available. The Plus got sold out quickly so not enough units available from Apple could be the only reason.