Character X is interesting, well-written, and tags out a lot. They're also very close with Character Y, often a major castmate or an in-game ship.

Character Y may or may not be bad in general, but they do next to nothing that's not threading with X or threading with a few others about things that end up being about X. So, anyone who wants substantial CR with X gets to deal with Y and with being turned into window-dressing for their drama.

The worst thing is that sometimes it seems like X-type outgoing characters naturally attract Y-type, migraine-inducing clingers-on.