In almost all the MMA fights I've watched, nobody seemed happy on their backs. Being good at BJJ can turn this into a plus if the opponent is a lot worse at groundfighting and is trapped in your guard, but even then, other positions seem preferable. If your opponent isn't trapped in your guard, god help you on your trip to ground-and-pound town. So to this day I don't really know why Kazushi Sakuraba was so afraid of pouncing on people on their backs. Hell, he wasn't afraid to ground-and-pound in their guards, but the moment they need a breather they knew to flop on their backs at a distance.
I do have guesses. People on their backs still have options: 1) upward kicks with knockout power, 2) legs to control the opponent's hips and legs in a variety of guards, 3) arms to grab their legs and trip them into nasty submissions. That last one seems to be what Sakuraba was most afraid of, judging from his frantic leaps out of his grounded opponent's arms and range. Ultimately, he would somewhat conquer this problem by spamming brutal kicks to his opponent's legs, but it would take a long time to do damage that way.
I'm not really satisfied with this explanation because I don't see other fighters have any issues pouncing on opponents who had fallen on their back. If it is actually an effective stalling tactic, I'd have expected to see it used more. Was Kazushi Sakuraba just an anomaly?