The thing I've noticed about Ravnica is that no one, in our group of around 100 players, is running a campaign, or even a One Shot, set in Ravnica. That's actually why I eventually decided against buying it - no one was interested in a campaign set there*.
The only thing that seems to have attracted any interest is the Circle of Spores druid. And it's worth noting that D&D Beyond (which is quite popular) allows you to buy that without the rest of the book.
Which leads to another idea - maybe these subclasses are not planned for a print book, but as a digital-only release?
* Eberron is similar. There is only marginally more interest in an Eberron campaign than there is a Ravnica campaign, but players are keen to embrace warforged and artificers.