I wonder what they would want to do with Dark Sun? Example: how can you not simply come up with some rules that would allow Psionics and the Kalashtar? Doesn't really fit in the current 5th edition rules, I know, but why should that stop the brain-trust at WotC from developing some rules on the fly for now?
Eberron (like Dark Sun and Dragonlance) is a different world. It was created with a specific theme and purpose: high adventure, pulp action, low-level but plentiful magic. These concepts, especially the last one, do not fit into the current rules. So, change them for Eberron. Allow a feat for all players at first level to allow for Dragonmarked characters. Allow Artificers to be creators of magic items, just not quite as easily as they did in 3.5-4.0. Make some new rules for 5th edition that only apply to Eberron races.
For what it's worth, I think there's a slight mis-step here.
You asked why they didn't provide a temporary "for-the-sake-of-argument" set of rules for allowing psionics (and Kalashtar) in Eberron.
I say: That's not an unreasonable thing, but it's "more". At some point, I'd rather have them release a glimpse of what they've got rather than hold it and keep working.
There's nothing wrong with giving me rules for psionics (play a bard/lore, wizard/enchanter or wizard/illusionist and flavor your powers as Powers of the Mind; your brain now counts as an arcane focus).
But there's nothing wrong with _not_ giving me rules for psionics, when focusing on Eberron. For me, Eberron isn't about the psychic, it's about the pulp. Psionics can be a part of that, but they don't have to be, and none of my stories center on them.
In short: I don't think they'd provide a UA for Dark Sun until after they'd tackled How to do Psionics For Real. Maybe they'd use UA as the platform for playtesting How To Do Psionics For Real. For me, psionics aren't a big part of what makes Eberron Eberron. For you, they might be.
That said, this doesn't hit my sweet spot for "plentiful low level magic".
I think for that, more than dragonmarks or even character classes, we need Common magic items.