The Realms changes include some great example of what I was talking about with "pet peeve changes". Someone on the FR team didn't like the real world analogues, so nations (and a continent) that fit that description just got removed. And new kingdoms got slapped in their place. Someone didn't like "fantasy Egypt" so Mulhorand went bye-bye. Which is the definition of personal taste. Some people are fine with real world analogues. Some people like them. And there were bound to be lots of gamers who set their campaigns in Mulhorand. Removing that nation was arbitrary. But someone on the design team decided their personal taste and preferences were more important than that of the people who buy the books. Which is slightly egotistical.
So, this is my tinfoil-hat, vaguely edition-warring conspiracy theory. You've been warned.
[sblock] By 2006 or so, WotC was sorely regretting having given up the pooch on D&D via the SRD. Part of the design theory that went into 4e was to make it as far from 3e as they could and still be called D&D. In essence, this gave them carte blanche to "fix" any problems they saw with D&D, damned the lore and tradition. The goal to was to make a D&D that couldn't be replicated using the 3e SRD. So they did. In the end, they created a game that no create could simply emulate well using the SRD.
But it wasn't just rules that got a makeover, its was story elements too. The regular tiefling was in the SRD, but the unified "devil-horn and tail" Baal-Turrath tieflings were not. Half-dragons might have been in the SRD, but dragonborn (and the associated concepts) were not. Hill giants might have been in the SRD, but the Earth Giants and Titans were not. Even some basic monsters got "unique" upgrades that were unreplicable using the SRD (like stormclaw scorpions) weren't. These also had the added bonus of being good fodder for their mini-line (as Reaper or any mini line could make an elf or dwarf, but the unique look of a tiefling or dragonborn could be a little more IP protected).
The 4e Realms was their first attempt at trying to reconcile the "new 4e paradigm" with a setting that was 20+ years old and firmly rooted in game concepts no longer relevant. So they effectively re-wrote the Realms (without making it a retcon) to fit "closer" to what was in the core-books. Dragonborn appear from out of nowhere. Tieflings all get the "Baal Turath" look. Eladrin over-write Moon and Sun elves. Gods that origins outside of Toril (like Tyr) get killed (and to be fair, a lot of FR deities do to; I mean they honestly tried to say Talos was really Gruumsh!?) And as part of the reshaping, any place that seemed like "real world culture X, in the Realms" got replaced by an Abeir portion (Mulhorand and Maztica most notably) in an attempt to make FR more "WotC IP focused" and remove elements that WotC didn't totally control (like the Egyptain pantheon chilling in Faerun).
Anyway, my point was I think a lot of the lore changes done both to the 4e Core and to the 4e Realms was done purely to shake things up and have a more "IP protected" version of D&D that couldn't be emulated by a third party and that they could exploit as their own unique versions. In the process, eggs were smashed and omelettes made. You can argue the omelette was tasty nor not, but I think one of the biggest drivers for 4e's changes was "different for difference's sake". [/sblock]
Rant over.