A matter of preference and the technology isn't there to see 4K at its best. I wanted a monitor with a higher pixel count than my last 1080p monitor but with a higher refresh rate than almost all of the 4K monitors I could find. 1440p hits the sweet spot. Plus, it's ultrawide.
My brother has a 4K monitor to edit videos and photos, and doesn't play any games. In my research, I found that they are wonderful monitors to generate creative or artful content, which I don't know a thing about and wouldn't do. My uses include email, word processing, doing Westlaw research, listening to music, and playing games a few times a week. I think that behind video editing, games are what normal people do that stresses a PC the most. So I wanted a PC that could run stressful games. This means getting a monitor with a high pixel count, at least 120hz refresh rate, IPS, and G-Sync modules. Like it has been stated in this thread, I did not find many 4K monitors that have all that and cost under $1,800. Even if I were to drop that much for a 4K monitor, there may not be a consumer-grade PC with a single GPU that can capably run a game without turning down more detail settings than an owner of a high-end GPU would want. DLSS may change my belief in the future. But as of now and after a few weeks of use, I have been impressed with the monitor I have. I didn't even consider getting an ultrawide. Now I don't think I can go back. Not all games/applications support 21:9 yet (are there many that support 24:10?), but there are hacks to make it work for a lot of them. I may have gotten lucky by getting a panel with negligible amount of IPS bleed, no flickering, and no dead pixel that my eye can detect. I heard this monitor, and several other 1440p monitors, tend to have more duds than normal in the quality control department.
I've had time to play 1 hour of GTA V with every setting maxed except I kept MSAA on 4X (not 8X) and grass detail on "high" (not "ultra" or "very high"). Everything else - extended distance scaling, all the shadow details, reflections, post fx effects -- maxed out. I ran a benchmark last night and all 5 "pass throughs" taken together resulted in a 73 FPS average, with dips in the low 60s in grassy areas and spikes in the 110s in the City. To me, that is acceptable-to-optimal performance. The game looks fantastic and runs without a hitch. Extended distance scaling maxed out and 4X MSAA probably combined to cost 50-65 FPS in most of the scenes. If I were to turn those off, I likely could see 120-145 FPS, which is what many reviewers did when they tested the 2080ti cards. This caused some people to get confused when they watched YouTube videos showing the card powering GTA V or the Witcher 3 or the Division at barely over 60 FPS. But either the uploader fails to mention, or people fail to ask, questions about the settings.
When I cranked up MSAA to 8X, turned grass to "ultra", and ran a GTA V benchmark, the FPS average went below 40. Out of further curiosity, I entered the Nvidia control panel and enabled DSR at 4X my native resolution. I went back into the game and ran a test at 6880x2880 resolution or something. ~27 FPS average. GTA V is a 3-year old game and has been fine-tuned for the PC as well as any console game can be. Yet this game can be extremely taxing at 1440p and with nearly-maxed out settings. With my monitor, I can almost get the best image possible and comfortably play at over 60 FPS. On a 4K monitor, I think I would need to turn down settings to get 60 FPS or keep the same settings and play at 35-40 FPS. That tipped the balance in favor of a 1440p monitor. Ultrawide just made the decision easier.
For fun, I fired up Assassin's Creed Brotherhood and played around 45 minutes. With max settings, I was seeing 220-300+ FPS, as recorded by the system on-screen display. It's a dated game (8 years?) that still looks nice. I exited the game back to the menu, cranked up the resolution to be 6880x2880 by again turning DSR on and at 4X the native. It was STUNNING how clear and detailed the faces, clothing, buildings, and foliage became. FPS went down to 80-90 (~70% performance hit). Personally, I cannot tell the difference between 60 and 90 FPS, so I kept it for that game. DSR really is a marvelous technology for older titles or titles where you have a lot of FPS headroom you can spare to get better image quality. I understand you wouldn't want to do that all the time, especially for esports/online competitive games where you need the high framecount.