We simply don't know what's going on behind the scenes. The impression I get is that initial development was rushed (big surprise) and they didn't build in enough flexibility.
That's not just "your impression", that's literally what they said, in no uncertain terms.
But they said that they were making specific changes/developments that would add the needed flexibility, and this was required to allow them to implement Tasha's, and that once they'd implemented Tasha's, implementing stuff like Supernatural Gifts would be a gimme (barely even paraphrasing). Then they did implement Tasha.
Of course that appears that the repeated assurances that they'd add other stuff and have flexibility going forwards because of these changes were
straightforwardly a lie. Or rather as businesses like to suggest "no longer true". LMAO. Good luck getting even your fingernail between those concepts.
D&D is a complex domain and is quite difficult to model in a way that it all works together, there's a lot of weirdness they have to figure out and testing any modification at all would be a bear.
Complex compared to stuff "normal" businesses (like my own) do with similar products? I work in "legal engineering" myself and nothing at all about Beyond seems complex. I've used products from companies with five employees that were dealing with far more complex and content-heavy rules-based stuff (and rules you REALLY can't afford to mess up, either!).
I'm sorry but it simply is not "complex" in any way that matters, and aren't you a software dev? It seems like you're aware of that. Official WotC D&D 5E just isn't that complex - nor was 4E. It's a relatively straightforward set of rules, which relate to each other in extremely predictable and well-bounded ways. The content they've point-blank refused to implement is well within those bounds - easy to prove because you can, manually, implement it, it's just banned to share the implementation. We were told, repeatedly that this was solely because of a restrictive licencing agreement with WotC. Now it appears that even with WotC in charge, and thus no licencing agreement in place, the same policy still applies, which undermines trust rather severely, given it appears to make them liars once more.
Now, if you were talking 3PP content, I'd agree completely. 3PP content often goes wildly off the reservation in terms of what it does with the rules. But that's not the case.
The only ones who could provide more details are on the DndBeyond team and they aren't talking for obvious reasons.
They used to talk a lot and accurately about upcoming stuff.
For the first 2 years they were actually pretty great at communicating, dealing with some thorny issues even, and maintained good info on what they were working on and likely timelines. This was whilst Curse owned them.
They started talking less when they got sold to Fandom, at which point it seemed like they were being pushed more towards making as much profit as possible, rather than proving a really good product with continual upgrades. Then shortly after they lost a bunch of staff who were the talkers, most of whom didn't get replaced. All the well-maintained info gradually slowed/stopped being updated, and previously-accurate info started getting wider and wider off the mark.
There was a lot of hope this would improve with WotC taking over, indeed I believe that was even implied by WotC (albeit circumspectly) at one point, but it's got significantly worse. Under Fandom the info about updates/changes did exist at least. Under WotC it's trickled off entirely.
So I pray WotC are just rather badly handling a change to a better product, but if that doesn't happen, eesh.