@Neil Arthur Hotep did a decent job explaining it, but I should probably explain it in my own words.
Think about buying a stock, the best time to buy it when you think the value of the stock is more than it is currently being offered for. You are interested in owning the stock so that you can gain the correction. You understand despite the market saying that the stock is worth X, that its fundamentals are that it should be worth Y.
There are a couple things that are true of all armies, number one is how much approximately 2000 points costs in models. Under the current point scheme without Start Collecting boxes for ease of matching, BoC armies are running about 600 GPB, while DoK for example are under 400 GPB. Yes, some armies cost more than other like SoB who are still only 480 GPB. Unless you are doing a very specific build 2000 points is under 500 GPB.
The next is how the army looks on the board. The game has been trending for while to smaller armies, and this has been doubly true in AoS3, which makes sense the complexity of the rule has increased having fewer units speeds up the game. Because of the low points BoC are a massive army on the board, basically consisting of an attack profile and a save. This also runs afoul of the CMD point mechanics which push players towards having fewer more impactful units.
There isn't a thing I can point to directly and say ah ha. But, given the context of AoS I think there is enough information to correct surmise that BoC aren't what GW would want them to be. Meaning they will be hammered into something more closely related to what they aught to be. This is probably the peak of BoC meat strength, and likely units will get more utility abilities to bring them more in-line with the design paradigm. Which means for the most part units that are interesting first, and efficiency second. And, as @Lord Krungharr experience confirms my analysis of the faction on the table, I think reversion to the mean is likely. Their points and recursion make them efficient, not interesting, unique, or GW's version of fun. And, it pushes them outside the established cost framework.
There is more, including GW and the secondary market but tbh its a bit shop talky and I probably should do some work.