Another reason? What's the first reason to upgrade?
Around 30% of iPhone customers are using an iPhone 5S or later. The iPhone 6 Plus is way too slow as it is due to it's lack of memory (something the 6S Plus fixed), so you'll see some of those customers upgrade.
6S users probably won't find a lot of benefits to upgrading, unless they're on a yearly upgrade plan with the Apple program, T-Mobile Jump! or AT&T Next.