Yeah it's a good question. I suspect in the event that the stoppage is coming as a fighter is badly hurt, they are usually to concussed and therefore function on muscle memory alone, which would be to tie up or backpedal etc. When a guy is clearly rocked but perhaps has his senses, it's often best not to take the knee I'd imagine, as the damage has already been done by the first punch. It must be something really drilled into their heads by most trainers etc. Bouncing back from a round where you were rocked is hard enough without being an extra point in the whole. Obviously a lot of it is just machismo and pride as well.