Whenever I want to learn something, I am highly motivated and want to start right away. If you make me wait until your next group starts, I have probably already found an alternative way to learn.
And the few times I did enroll in a cohort-based training I had huge difficulties following the pace of the course. They either go too fast or too slow for me, which is really frustrating. My weeks are irregular and not all your material is equally useful, so I would like to accelerate and slow down at certain moments in the course.
I understand the motivational benefits of going through a course together with a group of likeminded people, but those effects can be accomplished just as well (albeit with a little more effort from the course maker) by creating a community around the course.