I found that cowbell, jingle_bell and bell_collar do not imply bell even though that would appear to make sense.
So I wanted to suggest that implication but I also checked to see whether those got rejected for any reason. So I found these two implication suggestions:
bell_collar -> bell https://e621.net/forum/show/202567
jingle_bell -> bell https://e621.net/forum/show/218381
Apparently they didn't get approved. But there are also no reasons given.

So no I'm wondering: Why are those implications not in place?

Same reason many other implications and aliases haven't been approved yet. There's a large backlog. Admins are working on it, but it will take time.