Presumably because they assume it suggests the setting is coherent and not generic and shapeless.
I have never mistaken ban lists for coherency, distinctiveness, or shapefulness. The vast majority of banlists
are all about keeping things as generic and shapeless as they can--by making it so you only play the Extremely Traditional Options, in Extremely Traditional Worlds, using Extremely Traditional Tropes. It literally is about
removing shapeful options!
Moreover, a setting can be coherent as well as generic, and can be both coherent and generic while also not being shapeless. Dragonborn were added to Eberron, and Eberron is definitely generic in the sense that it was
built to allow any of the default classes to appear, any of the default
species to appear, and make adventuring a thing people are used to seeing in-world, sot hat parties of adventurers are a reasonable thing for people to be. Eberron was designed to be welcoming to any of the things D&D furnishes the player with being able to do/be. Yet it is deeply beloved specifically because it is both coherent and highly well-shaped, with tons of interesting locations, concepts, lore, and adventure hooks.
A ban list doesn't tell me a damned thing about whether the setting is "coherent" or not. Great example: Several GMs I've spoken with have banned non-"core four" species options (that is, anything beyond human/elf/dwarf/halfling) with various sociopolitical excuses, only to then do absolutely
zero thinking about how the ridiculously long lifespans of elves and dwarves (700 and 300 years, respectively, IIRC) would
completely warp the social fabric of any society with large populations of these species. Remember, if there were a 500-year-old elf in the United States today, that person would have been alive
before the coronation of Queen Elizabeth. A genuinely just middle-aged elf today could have
personally attended Shakespeare's plays. Can you imagine if we still had to deal with folks who saw African-ancestry people as "Blackamoors"? If we had to deal with the social and political fallout of conflicts that happened before any current government existed? (Remember, the United Kingdom only formed in 1801--the United States is actually one of the oldest governments on Earth.) 99.9% of the time when I see banlists like this, it has absolutely nothing whatever to do with even the slightest effort at "coherency", and everything to do with "ew, I think that's icky because it's unfamiliar to me".
(Note: at best I have a leg in this camp; usually if I have a fantasy setting that I'm closing off options I'm also adding some, but I still understand the position).
And I applaud that! Additions tell me
so much more than subtractions ever will.