Whatever the process for black blades creation is you can't reduce it to "others weren't interested while Ryuma, Mihawk and Zoro yes".
This would be terrible writing...indeed this isn't a real """explanation""".
It's nothing but a coping mechanism and not a real explanation, because if that's true it would mean black blades' hype is somehow empty.
In reality the process that leads to the creation of a black blade will be some serious shit that will make anti zoro pirates cope even more