So instead of a slow release schedule why can't some of you just have a slow purchasing schedule?
I am hardly an experienced voice on this topic, it here is my take. For a while now I have been thinking of getting into the Final Fantasy series of games. Never played one, but I know they are heavily story driven, so air want to do it in order. That amounts to so many options to choose from, and it is incredibly intimidating. The same thing would happen with this, I think.
A fair number would get intimidated by the wall of books that shows up when they Google "D&D 5E". According to their friends, this is the easiest edition to learn yet. So simple, and so "New player" friendly. If there were even 30 books for it before I started, I would probably have been put off by it a bit, knowing how many hours of content can come out of a single book. Considering a portion of the market they were targeting is new players like me, I think that would have put a significant dent in their goal.
Lastly, if they paid for hundreds of thousands of copies of a dozen books a year, and most people only bought one or two, while something like a couple thousand made sure to buy every book, that would be a massive money sink. According to some people that is essentially what happened to a previous edition, so I can understand them being more cautious.