If Apple gave the iPhone 6 Plus a significant spec bump over the iPhone 6 (like if the 6 Plus got the triple-core A8X CPU clocked at 1.5GHz, octa-core GPU, and 2GB of RAM vs the iPhone 6 with the regular dual-core A8 CPU clocked at 1.4GHz, half the GPU cores, half the RAM) it would segment the companies flagships way too much. People who got the iPhone 6 Plus would get all the way to iOS 13 or something while the iPhone 6 would only get to iOS 11 or 12. You'd see apps that would support iPhone 6 Plus but not iPhone 6. So people who want a regular size phone would technically have to get a 1 year old device. If they want a device with more longevity, features, and power, they would HAVE to get a huge, more expensive, phablet. People would be really upset by that.
I'm already upset enough because of how the iPad mini is always given inferior specs, I would hate to see that start happening in Apples phones as well. I think the full size iPad is too big, plus it's a lot more expensive than the mini. Since I prefer the mini size I have to have all the good specs and such taken away from me, I hate that.