I think it has to do with the fact that human minds and brains are different. Also, the time in which we find ourselves is different, the technologies (both meditative and of the more garden-variety) are different. Basically, everything is different, changing, and it seems that often things build upon other things.
One person (or persons) designs a better map, because they become aware of the limitations of the previous one and want to address those deficiencies. Then we get new information, and realize problems with the new map. It's really a dialectic across time, with human minds and hearts trying to continually close the gap between experience and their mental models of experience. I don't think that gap can be closed, given the nature of human experience, but that's more a philosophical question than anything. I'm just glad that I've inherited the beautiful and helpful models and practices that I have.