what kept yoda from going with luke to bespin?

Yoda wanted to train Luke to be a Jedi. If Yoda had gone to Bespin he would have been killed.

It would've been too hard to film him.

Like heights, I do not. In the clouds, have difficulty breathing, I do.

The story was about son confronting his father. I liked the fact that Vader (and the emperor) had no clue that Yoda was still around and believed till the end that Kenobi teached Luke.