It is my understanding that kata oyo, and the bunkai process by which you derive them, weren't taught all that much by Funakoshi Gichin, and which were almost entirely left behind by Funakoshi Gichin's son and his contemporaries, so that by the end of WW2 pretty much everyone doing Shotokan wasn't learning oyo or bunkai. After WW2, the Okinawans needed to rebuild, and teaching karate to the soldiers stationed there was one way to get the money for that, but many of those soldiers had friends stationed on mainland Japan learning Shotokan, and so that's the sort of karate they wanted to learn. At least some of the Okinawan instructors during that era stopped teaching kata application because their students simply weren't interested in it. They also tended to issue those soldiers yudansha grades between 1st and 3rd Dan during their tours of duty, with the expectation they would come back to Okinawa to continue their education, and that's probably where the "bunkai is for black belts" thing really came about. Most likely, those soldiers just didn't learn kata application while stationed on Okinawa, and only picked it up later, and so they assumed it was black belt material, or they made their students wait until black belt to learn it so they could market it as some sort of "secret knowledge."
Now, personally, I think this is a terrible approach, even as an attempt at retaining students. We all know that most people who train in martial arts who make it past the first year will still end up quitting when they earn their black belt, and that happens regardless of where you stick the "secret knowledge" of your curriculum. All this ends up doing is producing a bunch of people who trained in karate long enough to earn a black belt who have no idea how to actually use the classical material of the art, which makes karate look watered down, outdated, and ineffective. On top of that, it is cheating the individual students out of valuable skills for self-defense! My late Sensei taught application right along side the kata, and I do so, as well. This has left just about every student we've ever had with not only a better understanding of the art, but more appreciation for it, and an awareness that there is more to karate than point fighting tournaments and Kyokushin.