i think you've hit the nail on the head regarding the rewards. they're great in the short term, but don't hold their value. mithrala is obvious; she's a one and done, so she stops being a motivator as soon as you have her.
stoneskin is pretty similar. it's objectively a great armor set, but how many champions do you really expect to put in stoneskin sets? i'm currently at one and don't expect to go much beyond that and i feel like most accounts are only going to have a couple of champs at most. once you have those few outfitted, you might occasionally swap out for a better piece but on the whole, stoneskin is no longer a draw. compare that to the gear from cb. i've got dozens of champions that use cruel and immortal sets and i expect that trend to continue for as long as i play this game.
protection is more viable since it's useful in all areas of the game, while stoneskin is pretty focused on arena. that said, i still don't see myself using it much although here it's more of a supply/demand issue. i get maybe 5 or 6 protection artifacts a week and 90% of them get sold imediately. to get the maximum value from protection artifacts, you really want a full 6 pieces on a champ and after almost 6 months of hydra i could maybe do that for one champion if i use a couple substandard pieces of gear.
now, compare the above (plus the clan xp and gold, which are nice but not really a huge draw) to the rewards from cb. the armor, as mentioned, is much more frequently applicable than either hydra set, but far more importantly i will always need more shards and more tomes. adding those into the hydra rewards would go a long way towards motivating people to hit it more frequently.