It's being reported that the iPhone 6 is outselling its larger brother by the 3:1 ratio. Setting aside how accurate these numbers actually are, let's take them at face value and try and figure out why there is this vast difference.
This is the obvious reason that hasn't been talked about enough. The iPhone 6 Plus simply isn't available at most retailers. If you walk into most stores today, whether it's a carrier like Verizon, AT&T, Sprint, or T-Mobile, or a big box store like Target or Walmart, you're probably not going to be able to buy a Plus even if you want to. What you will find at each of those locations is the 16GB iPhone 6, and maybe the 64GB model if you're lucky. Yes, the Apple Store probably has some Pluses in stock, but most people buy their phones the same way they always have, subsidized at a carrier's store.
I can personally attest to this challenge. I currently own an iPhone 6, but I was really looking for the 6 Plus. I eventually gave up looking after a few weeks because I simply needed a new phone and settled on the smaller device. Am I happy with it? Absolutely! Does a little part of me wonder if I would have liked the Plus more? Sure, but I'll live.
Among my friends and colleagues, it's the same story. A lot of them are intrigued by the Plus but simply can't buy it without getting put on a waiting list that basically says you'll get one between now and several months from now. That's no way to buy a phone and I don't blame them for saying, "that sounds ridiculous, I'll just take the regular iPhone."
Then there's the price. At $100 more than the standard issue iPhone, the 6 Plus is a more substantial investment than most people are prepared to make on a phone. For many people, paying the $199 for a new phone is already a bit of a stretch, so asking them to add on top of that is a hard sell. Other large phones like the Samsung Galaxy Note 4 also sell at a premium over their smaller counterparts, which is a big reason you see way more Galaxy S5's out there than Note 4's.
Smart people with means will tell you that $100-200 extra for a phone is not that big of a deal when compared to the entire cost of owning a smartphone (the contract will cost you thousands over the next 2 years), but for people who are not as well off, that's a tougher pill to swallow up front.
3. Bendgate (sigh)
I hate to bring this up, but this is what I'm hearing from "the man on the street." People really think that every iPhone 6 Plus bends. Every damn one. Obviously that's not true, but the perception is out there, and I think it's hurting the Plus's sales at least a bit. We can look at Consumer Reports' testing to see that the phone is not as fragile as some people fear, but this stupid video has over 57 million hits and is all the research many people will see.
I think that if you could walk into a store, any store, and they had every model of iPhone 6 and 6 Plus in stock, the numbers would be much tighter than they are. As I wrote earlier today, the only month the Xbox One outsold the PlayStation 4 in the past year was the one month where there basically zero PS4s on store shelves. If the iPhone 6 Plus is less available, more expensive, and has a (dumb) _-gate controversy, then it should be no surprise that the phone is selling worse. I don't think it's at all a fundamental failure of the larger form factor at all.
Maybe next year Apple will have their production in better shape and we'll be able to buy whatever phone we want wherever we go and the numbers will level off. I'm sure this will still be a topic of discussion and contention in a year, so we'll definitely find out.