Honestly, price. I've had my XS Max for a year and if I sold it now, I would have paid around $500-$600 to use the device for one year. I'm waiting until I can get the price around $350/year or less before I upgrade again. Based off of calculations/trends I've used as far as how much iPhones depreciate per year and how much I should be able to get for selling my XS Max, I probably won't upgrade for another 2 years. The longer I keep the phone, the more money I save.
If I keep my XS Max for 2 years of total ownership, I should save 38% if I sold it after year 2 vs year 1, coming out at $371/year. If I keep it for 3 years of total ownership, I will have saved 53% than if I sold it after year 1, and 15% than if I had sold it after year 2, costing me about $315/year to own. If I sell it after 4 years, I only save 10% more than if I had sold it after year 3, coming out at $283/year.
Additionally, the upgrades this year were great, useful, incremental improvements, and they'll be great to have on the next iPhone I get, but they were none that would really change my phone usage that drastically day-to-day.