It’s probably both an oversight (in that they didn’t appreciate that Bane would be basically useless in TB when they designed his kit) and something they’re not going to change (because it would disincentivise gearing characters to some extent if missions could routinely be run with less than 5 characters).
What they could do, of course, is create a two Sith special mission for Bane on one of the planets. We know they can do that from the Cal/Cere mission on Bracca. This would be well received by all, I’m sure.
Final piece of TB trivia… on Kessell, the Combat missions can be run with only 4 characters (other than the Jabba mission that needs 5). I think they are the only character missions where you can launch without all slots filled.