There are really just a few reasons, but I freely admit that it has a lot to do with how much I like the 7 plus. However, here are the top 5 reasons:
1. 1st Gen Unforeseen Issues - This isn't my first rodeo. I have seen Apple come out with new products and many of them have unforeseen flaws (there is already talk of screen burn-in).
2. FaceID - I know I will eventually be forced out of TouchID into FaceID if I continue buying iPhones. I just don't want the inconvenience when my 7 plus still runs great. Eventually, the 7 will feel slow and I will want a new phone, but there is just not enough reason to go to a technology that needs to be stared at and the swiped compared to just touching a button.
3. The Notch - While the point of shrinking bezels is something I completely understand. I also understand why they need the notch for FaceID. However, it makes the screen smaller in landscape mode or it cuts into photos/video in landscape mode. I prefer the constantly larger screen of the 7 Plus.
4. Screen too small - As I said above, I prefer the 16:9 size of my 7 plus versus the 19.5:9 aspect ratio of the X. If you watch a 16:9 video in native format, it becomes a 4.96 inch screen when compared to the 7 Plus 5.5 inch screen. If Apple makes an X Plus next year or 11 Plus (whatever they call it), I will be much more interested, because even with the notch and longer aspect ratio, it should have more active screen space.
5. Price - Given the reservations above, I just don't want to spend the money to get one right now. Also, because of the glass back, Apple Care is almost a necessity, which only adds to the already high price.
I am not saying there aren't some positive attributes, but for me, they aren't enough to get me to trade in the 7 plus this year and pay extra for the X. That being said, I will be much more interested in a plus sized version.