I mentioned this in another thread on this subject, I used to self suck. Suppleness is important but really it is age that is more important. Of course older men who are slim with a shallow build can continue to self suck, but it is more usual to get heavier, and thicker in build as you age. A gut doesn't help. The vertebrae in your back start to stiffen and fuse with age especially in the lower back, that restricts your ability to bend into shape.
I don't understand the big deal. I suppose if you are rather fat, not supple and with a shorter than average erect penis, you won't be able to self suck, and of course if you are older. Put it to you this way, I know what my penis tastes like, what it smells like up close, and I know how my penis feels like orally. It depends on your ability on how much of your penis you can get into your mouth, it is difficult to get beyond the glans to the shaft, and is only possible for the truly double jointed. You need a comfortable chair which will enable you to comfortably curve your lower end over your front end, the shape of the chair helps. Anyway I could go on, and I am getting nostalgic for the days of my youth.