The core problem - which Kennedy realizes but so many other do not - is that Johnson (and McCarthy before him) has to walk a path that doesn't lose him either a half dozen conservatives or a half-dozen moderates. That path is very narrow, and on some issues, no such path exists.
Hardliners call him out for not "hanging tough", but the positions they want him to take would lose votes on the other end. And what the conservatives don't seem to realize is that while they have no alternative, the moderates do. It's the raw nature of the two-party beast that the people in the middle have the most sway because they're the ones who can sometimes get much of what they want from either party.