One other thing that makes co-ops so undesirable is the time commitment. To do a 3-player co-op, each player has to commit their attention and armies to the adventure at the same time. This takes a bit of coordination, and everyone has to put the effort in at the same time. Compare this to BK or RB, where each person can run independently and invite only at the very end.
Not counting troop costs, three players could coordinate times to get together and run one 3-player coop and get one set of loot each. OR they could arrange to run 1 BK/RB each and swap lootspots and get 3 (better) sets of loot for about the same amount of time, effort, and coordination.
As long as co-ops are way more hassle to run and give way less loot for the effort, they will continue to consistently be deleted from stars rather than run.