I've used Ubuntu as at least a dual booted daily driver since 2016 and have also discussed with friends and family about what they liked and hated about it when they have used it, how they use their computer and whether they would swap. Here are some observations from that:

Hardware Issues and stability: For the most part, I've not really had to deal with hardware issues outside of trying to get NVidia graphics cards to play nice with everything else. However, I often have weird system stability issues or just plain quality of life. E.g. 2-in-1 decides randomly when put into sleep mode to flip the screen to a random orientation which I then have to go into settings to revert back. I'm used to buggy and annoying software, but for a lot of people this is a complete killer. Similarly, while I love the diverse options within Linux, having so much diversity means that troubleshooting and testing is so much more complex and you will usually have to go over multiple answers in order to solve your problem making it much harder to get into and use reliably

MS Office: This one tends to be the largest reason in my experience for people not going over to Linux. For a lot of people this is their main use for a computer and the fact that it is not available on Linux is a deal breaker. I've tried the online version and it is just not a viable alternative (nor is any cloud option). Similarly LibreOffice is a lot better than nothing, but the UI feel like it came out of the 90s, Latex is faster and easier to use than the math input, I never have been able to get referencing to work, drawing tools are lacking if they even exist at all. Opening office documents breaks all the formatting and looks awful, etc etc.

Games and other windows software: While I think value has done wonderful work in encouraging developers of games to support Linux and Proton does work quite well, you never quite know how a game will perform on Linux and if it will even work, whereas on Windows you can guarantee it has been tested and will work well. Similarly for other software: Will this work on Wine or Crossover? Maybe or maybe not but it's a bit hard to swap if you are paying large amounts for software just to find out it cannot run on Linux.

A reason to move: I think Linux will always remain fairly niche as for most people there just isn't a good enough reason to move over from Windows or Mac. These platforms already offer them everything they want in a computer in an easy to use and polished way. For most people, they really don't care that windows is constantly spying and with ads everywhere already, what is a few more ads or that the cost of a Mac and is absolutely extortionate. Moving across would require a whole bunch of troubleshooting and learning how to do just about everything all over again and that would require a really good reason to do which Linux doesn't (and possibly can't ) provide and MS and Apple haven't done anything stupid enough to offer.