My guess is that it started as a desire to have a thematic counterpoint to the Tiefling in the PHB.
Once they decided to add (back) in the "Choose your Planar Origin" aspect, they wanted an "Aasimar" type counterpart that chooses between Upper Planar options like the Tiefling choose between Lower Planar options, but they'd only just recently reworked the Aasimar in Monsters of the Multiverse and changing Aasimar "subrace" options to "Pick an Upper Plane" might mean losing the cool "Fallen" Aasimar variant. So they decided to tie Aasimar more explicitly to Angels (rather than Celestials as a whole) and created the Ardlings to pick up the ties to the rest of the Celestial races, then made the Ardlings more "bestial" to help differentiate them from their Aasimar cousins, as well as to represent ties to more "bestial" Celestials like Guardinals, Hound Archons, etc.
But, rather than seeing the Ardling as another type of planetouched like Tieflings, people fixated on the "animal person" aspect and saw them more as a unified Aarakocra/Tabaxi/etc. framework, and so that's the direction the design team drifted in as well, ditching the broader Upper Planar themes and tying them specifically to the Beastlands for the second pass.
Presumably, at some point, "animal person" became more core to the Ardling than "celestial planetouched", and thus it stopped serving the role they had intended it to in their initial design, so they decided to pull it and recalibrate.
I mean that they wanted multiple options to choose between for that "big guy" visual archetype.