I don't think there's anything particularly great about it, and it's notable that actually relatively little pre-D&D fantasy fiction was set in medieval-Europe analogues.
If you look at Conan, that's really a version of the ancient world, not the middle ages. Fafhrd and the Grey Mouser has more of a 500 BC or 600 AD vibe than an 1100 AD one, and more of a Byzantium or even Babylon vibe than a London one. It's a mish-mash, but it's not "medieval Europe". Lord of the Rings has a lot of early-medieval stuff going on, but it's still not really "medieval Europe" (it is a bit closer). Michael Moorcock's stuff is usually ancient world-styled, in different ways, with only Von Bek (which I think was quite influential) really being "medieval Europe" (albeit really, really late medieval/renaissance/early modern) or outright steampunk-before-steampunk. Vance is far future post-apocalypse and definitely not medieval Europe. Lord Dunsany's fairytale stuff is a bit more medieval, but still, not really, to my mind. Lovecraft and his imitators obviously aren't, nor is Zelazny.
But it seems like the wargame(s) that lead to early D&D were centered on medieval warfare concepts, that expanded out into fantasy.
A lot of very early stuff seems to have a non-medieval feel, a sort of ancient world or fantasy + science deal (Empire of the Petal Throne, 1975, was very non-medieval though I guess technically not D&D), but I think it's Greyhawk and similar which pushes towards a much more medieval take, with it's obsession with heraldry, knights, and so on. And more and more medieval stuff gets added, and non-medieval stuff gets shoved out as time goes on. Because I think that's the stuff people are familiar with - I get the feeling there were a lot more books on medieval history, medieval warfare, medieval weaponry and so on, on the shelves of the people developing D&D, than ancient world or non-Western stuff (which is actually kind unusual in the period, maybe). I mean, when you're making distinctions between a dozen ridiculously specialized polearms (most of which I love but come on...) but aren't including non-Western weapons, that's a clear sign of medieval influence being strong on the rules, rather than the broader fantasy influence of earlier.
So through the '80s this seems to get stronger and stronger, and by 1989, we see the 2E PHB, and there's no Barbarian, and no barbarian-style artwork, and tons and tons of ultra-medieval stuff, and virtually all the artwork depicts a Western-medieval fantasy world, and Forgotten Realm is just that. Admittedly there is still some deviance - the first setting for 2E, AFAIK, is Taladas (Time of the Dragon), which is not Western-medieval, it's Dark Ages Eurasia (ERE and WRE equivalents) + lots of weird crazy stuff (steampunk non-moron gnomes, Dune-esque sailors on a sea of glass, tons of Pacific Islander-related stuff). That said I think the initial 2E era is the peak of "medieval-ness" in D&D. As soon as more material comes out, it starts to slide away from that.
So I don't think that there was ever a conscious design decision, with D&D, that it is "medieval europe" primarily. I think it's an accident of the interests of the main designers and writers in the 1970s and 1980s, and had a few people had different interests, we might have seen a rather different take (probably more ancient world-y, less medieval), which would in turn have influenced fantasy writing, computer games, and so on.