It's in the name.
More seriously, after reading all 14 pages of posts, I actually want to answer the question the OP asked, which is why
I prefer a "medieval milieu" for my D&D. Firstly, I want to start by quoting from the Mentzer Red Box (emphasis mine):
"Imagine: it is another place, another time. The world is much like ours was, long ago, with
knights and castles and no science or technology - no electricity, no modern comforts of any kind.
"Imagine: dragons are real. Werewolves are real. Monsters of all kinds live in caves and ancient ruins. And magic really works!"
To me, those two statements are the very essence of what makes
Dungeons & Dragons what it is. The emphasized part is what defines the era, and I'd like to call special attention to "no science or technology - no electricity, no modern comforts of any kind." To my mind, that bumps us to before the "Age of Enlightenment," which means Renaissance, Medieval, or even earlier.
But why "Medieval?" Some of it is that was the setting of the fiction and stories that inspired my love of the fantasy genre. It wasn't
all medieval (both Renaissance and pre-medieval stuff was there, and Howard and Leiber are as weirdly anachronistic as Tolkien), but if I accept the premise that the true fantasy aesthetic (for me) is "no science or techology," I start thinking about a kind of fantasy pre-modern "Neverwhen." If the fantasy world I'm imagining is an ancient one, I'm looking at having instituted a sort of "technological stasis" (as Tolkien did).
And the longest period of real-world technological "semi-stasis" (not
actual stasis, mind you, but if you squint, it can kinda look like it) lasted from a few hundred years into the iron age (let's say ~600 BCE) to the start of the Renaissance (~1450 CE), which is like 2000 years. That's a full two millennia(!) where steel weapons and armor were in use. That conveniently includes a fallen empire, knights, castles, and a world where many people still believed in magic and monsters. And the medieval period (early to late middle ages) accounts for almost half of it.
After 1450, I can notice (looking back) that the introduction of firearms led (by 1700) to the end of armor. And a couple hundred later, increasingly better science gives us electricity and the rest of the modern age. So while the basic tropes do still mostly hold in the Renaissance, it's hard for me to imagine that a Renaissance-era world (technologically-speaking) could remain in any kind of technological stasis - because it didn't. So I dial the technological advancement back to the last era I can justify things having "paused," which is the "Medieval" period.
But I can acknowledge that
technological stasis doesn't have to mean
sociological stasis, which justifies a lot of the anachronisms. My brain also goes back and forth over things like the printing press. And of course, magic (even rare and expensive magic) would definitely change things, which is why I generally prefer settings with less reliable and commoditized magic. Because they give me less world-building to do.
Eberron is fun, but it's closer to "Pulp Adventure D&D" than the traditional kind.
And that's why I prefer the Medieval Milieu for my
Dungeons & Dragons.