Maybe one of the problems is that most of the people who can afford these machines have full time jobs that don't leave them enough time to get into it. When you first look at how they work it seems almost as if it should be as easy as operating a printer: plug in the design and load the fabric and thread, then hit go. However, there is a whole lot more to learn, and the thread and stabilizer you will need cost a lot more than typing paper.
The other aspect of it is that it can seem rather boring because after you set it up you sit back and just watch it stitch. You have nothing much to do except change the thread as each color is sewn. Many of the more elaborate designs can take a long time to stitch out, and if you happen to decide it's going well and you can maybe duck out for a cup of coffee or a pit stop - Bam! These machines somehow know when you have turned your back and that's when something will go wrong every. single. time. You'd better hope it's just a thread break and not your beautiful, expensive, almost-done project jumping out of the hoop and being destroyed or possibly damaging the machine. (You can and should pause the stitching if you can't be there.) In short, this is not a process that will appeal to everyone.
If you are considering buying a used one, this is one thing you should get from a shop because the warranty and instruction that a shop can give you are important for safeguarding your investment and helping you make the hobby fun rather than frustrating. At very least be sure there is a nearby shop that will service it because shipping it off to be serviced would be very cumbersome. Also, before you buy look online for video demonstrations of the machine you're considering and also try to find a pdf of the manual and read it so that you will know how to test it and how to check the "mileage" before you buy.