Do we know exactly why retention is so low?
I think that we actually sort of do know.
And while those things you listed are for sure contributing factors, most are not really serious issues, let alone deal-breakers.
For instance, CB is not really that complex at all, but yes, the interface could stand some improvements, but not to reduce the already lacking complexity. If anything, due to the ever-escalating bonus value of the NUB, people get pretty big rewards earlier and earlier. The lack of a clear goal is not really an issue, seeing how there's so many thriving games out there who are even less goal-oriented. And the wiki is not really that stuffy, you should see some other wikis of other games I casually play. The text-only aspect could be a turn-off for several people indeed, but it's not such a tiny niche to turn off nearly every new visitor, and there's more than just a few surviving MUDs out there too (and that's even more text-based than CB).
The reasons people run away from CB very fast ?
First and foremost, the NUB/NCB system has bad side-effects and is getting worse the higher the bonus value goes in time (and it keeps going up due to the way it's set up).
The higher that value goes, the more annoying it gets to find decent early targets, as you go by targets faster and faster, and the target selection mechanism SUCKS in general, let alone for non-supporters.
In fact, one of the very first "band aid" things we should probably do is to give all CURRENT supporter targeting features to all players from the word go, AND scramble to improve the means to more quickly seek out potentially reasonable targets.
The score system is sorely lacking, it is a poor distorted reflection of actual performance, it's like seen through carnival mirrors. It should be redesigned to more closely indicate actual fighting potential ASAP. This also feeds back into the previously mentioned issue.
The lack of new developments becomes quickly evident, both through the lack of meaningful change logs, the lack of activity and general negativity of forum threads (which can't really be helped unless you want to start a systemic lie campaign which people would quickly uncover either way). If something scares off people really fast is a game that "SMELLS" like it's in palliative care, and CB sure does smell like it. That won't get "fixed" unless we can find and empower people with enough spare time to actually develop the game as a main activity, which is, well, not that easy.
And last but not least, there's a horde of legacy Jon-mandated decrees which probably made sense many years ago when he still had at least one of his hands on the pulse of the game and he kept on sort of actively developing it, but the insistence on maintaining the status quo which is obviously not working anymore is the finishing coup-de-grace to the horribility basket of nuisances. If we wish for CB to have a second chance, a lot of them need to go the way of the dodo, and they need to do that FAST.