Because training takedowns for a competition doesn't really make any sense when you think about it.
Takedowns are the only aspect of BJJ competition that is optional.
You have to know how to work from the bottom because you might get swept. You might have to know how to work from the top because you might get caught in guard. You gotta learn how to escape because someone will eventually put you in a bad spot.
Takedowns, you never have to do because you can always opt to just sitdown. People say buttscooting is a foul but there are ways around it.
If you don't believe me just google bjj competitions. I would say only 1 out of 5 or more even have TDs attempted. At any level.
Add to the fact that TD's are painful to train and more dangerous, not to mention that you can easily get to a very high level in BJJ without have much TD knowledge at all, and it's pretty easy to see how you can go a long time without an instructor even touching on them.
I think you are much more likely to see a Judo guy with good subs than a BJJ guy with solid takedowns. If anything, the percentage is the same.