I think what you are asking is why people fail to stick with CPAP treatment enough for it to be effective?
I can share my own personal difficulties with it, but I'm sure it varies. For starters there was the discomfort of using the mask. The first time I used one they gave me a full face mask in a lab setting and my anxiety set in and caused me to have a panic attack. I managed well enough to sleep for a few hours but it was an awful experience. I didn't try again for 8 years and then out of sheer desperation.
The first time I really gave it a try I was only successful for about 10 months and gave up again. The smaller nasal mask was much more tolerable to wear though which helped my initial issue. One of the major difficulties I had was chronic sinus infections. Whether they were caused by or aggravated by the treatment I really don't know but I couldn't use it when my nose was blocked or when the pressure of the machine caused such horrible pain in my sinuses.
It also dried my eyes out very badly to the point I was getting awful infections from irritation with my contacts so I had to switch to glasses. I had trouble sleeping with my mouth closed even after using a chin strap. My lips would open and cause noise and it would wake me up. I ended up having to use both a chin strap and tape my mouth closed at night. Not a very comfortable way to sleep. And between the mask, chin strap, and tape I had terrible lines all over my face that were quite embarrassing. For that matter wearing all of this in front of my husband was also embarrassing to me.
I had trouble with bloat now and then. I'd wake in the morning with my abdomen seriously distended and awful pain from it. Then painful gas that would last much of the morning. At the time, I was having trouble with my gallbladder and the added pain and pressure was awful.
Beyond the physical issues, I also found it a pain to deal with. To remember to clean the parts and have it set up and ready to go. I had to make sure my face was cleaned before using it too and by the time I got my kids to bed at night sometimes I was too exhausted to do anything but crash. Or I'd sit down to have a quick snack and fall asleep not meaning to.
I also didn't find it helped my fatigue all that much. While I always have far fewer headaches on treatment I'm still pretty tired most of the time and I still wake up a lot during the night. So the benefits were minimal.
After being off for 2 1/2 years I've been using it again for about 6 weeks. Hoping to stick with it this time around, but time will tell.