Maybe because there aren't any 6+'s available for T-Mobile. They don't even have 6+ display models out in their stores. I went to ATT and saw a 6+ on display but the lady said they didn't have any to sell, and probably wouldn't until December.
Trust me, I already looked and called all over for this phone. Apple simply can't make enough of them, fast enough.
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Actually, Google said they would support products for 18 months (minimum), and 'could' continue for a longer time period depending on external factors, like hardware limitations.
The original Nexus 7 is now over 24 months old. Be grateful it is getting 5.0.
Actually, I don't think the manufacturing is the only problem. I talked to an Apple rep friend of mine and he said both UPS and FedEx can't keep up. They would need to increase their capacities (and allocate it all to Apple) to handle the orders Apple is trying to send through them.
Apple had a small cargo airport built next to their big manufacturing facility to get them out as fast as possible.
I don't think people really, truly understand the sheer volume Apple makes and moves in a relatively short amount of time. Sure Samsung sells more devices each year, but we're talking dozens of products released over the entire course of the year.