While I've never set a game in the Forgotten Realms, I totally get why it's a good base setting. It's flavorful enough to be able to say, "oh yeah, Waterdeep and Baldur's Gate, those are in the Realms," but generic enough to not jar new players' expectations. All of the Forgotten Realms' basic concepts are in the core rulebooks, and pretty much always have been.
Eberron, while my favorite setting, has a bit of a departure from classic D&D. Dragonmarks, warforged, kalashtar/psionics, those are all things that haven't been in the core rules. And they wouldn't fit in all settings. Magic robots and people that are closer to Jedi than wizards aren't exactly what I'd call iconic for D&D.
Dark Sun is way too weird for a "default" setting. Cannibal halflings, elves that are ridiculously tall and raid/enslave folks, precisely zero divine magic, and Preserving/Defiling are huge deviations from the D&D norms.
The closest setting that could compete with Forgotten Realms is the Nentir Vale, since it was built from the ground up to include dragonborn and tieflings. Personally, I'd have stuck with it, but that's just me. Choosing the Realms made sense. It's hugely popular (relatively speaking) because of the novels. Drizzt alone is a hugely popular character.
Really, my biggest issue with the Realms is that they basically hit the reset button. The spellplague and all the other huge shakeups in the 4E era actually made the setting interesting. The Neverwinter setting book was fascinating. If ever I happen to run a Realms game, that's likely the era I'd put it.