I think the issue goes back to the original view of how the game of 5e was being developed. Costs had to be managed, kept as low as possible, because the expectation quite literally was that this was it, and the game would just stagnate after.
Once it became clear that wasnt the case, its been a matter of 'dont break anything, what can we release without breaking this unintended lightning bottle'.
Those of us who love the concepts of Planes, and Gods, and the structure and relationships between them? A fraction of a fraction.
Its just not been worth the time/money.