Just like all the sports like this in the Olympics, the sport has its particular rules.
In my opinion, it's because most judo people look at judo from the standpoint that judo is an Olympic sport and governed by the International Olympic Committee. So when you are on the ground in judo, you have a short time to make it happen before they stand you up. Just like all the sports like this in the Olympics, the sport has its particular rules. Jujitsu is a grappling art. Judo is a throwing art with time limits on ground grappling, then we stand and try to win by ippon. For you own interest, you would pick a school that does what you want. Judo grappling is based on a missed throw, to choke, pin, or submission. It seems jujitsu is all 100% win by grappling and no point for a throw. This is only my opinion with 5 years judo, 4 years karate, 5 years boxing, and 7 jujitsu lessons.