I think the problem is that it's custom loop money, and that puts a lot of people off. Those wanting an AIO don't want to spend so much, and this is, for all intents and purposes, simply an AIO, albeit a very good one.
Anyone with the money starts to look at a custom loop and realises the potential that offers, in terms of both aesthetics and better cooling. Yes it's certainly convenient and maintenance free, so by all means will appeal to a certain type of person, but is rather niche I feel. Also, I think EK lost a huge amount of favour as a result of all the leak issues with the Predator, and while I've not seen any such reports with the Phoenix, it does give pause for thought before buying something so expensive. The lack of reviews is more puzzling (and would aid sales I'm sure), but perhaps EK just aren't sending them out to reviewers... and given the cost, not many will dip in to their own pockets for one.