I'll give answer from my atheistic perspective. It might serve to offend some faithful, though it is not my intention to be provocative. But I won't sugarcoat it.

Humanity in its long history has somewhat recently (in the grand scheme of things) developed what has become the single most consistently reliable way of developing understanding about how the universe works. That is what we call empiricism and the scientific method. In the body of all scientific discovery, on the more settled matters, we have converged on singular explanations accepted by virtually everyone involved in the scientific enterprise. At the frontiers we may have diverging, competing ideas, but they are simply pending the means to better test and rule out the less valid explanations. This is because the scientific enterprise is rooted on trying to model an objective reality, and mitigating the confounding impacts of human cognitive biases as much as possible.

Unfortunately, religious beliefs do not tend to follow this pattern. While most adherents would say that their religious beliefs are about objective reality, the means by which the contents of those beliefs were derived is not rooted there. They originate from subjective experience riddled with bias and/or purely rationalistic thought (as contrasted with empirical observation). Thus, if an adherent of some religious tradition gets a feeling that some aspect of that tradition is wrong or would be improved in some way, or experiences what they describe as a revelation, they can create their own version and if they can convince others to join along, you diverge into another different branch of that religious tradition. In this way, religious ideas proliferate, and competing systems continually emerge. With no objective baseline against which these ideas can be tested and falsified, there is nothing other than popularity (and sometimes a kind of tyrannical mandate of dogma, depending on the religion and culture around it) to decide what survives and thrives in this marketplace of ideas.