I'm currently playing in Mystara. I pitched it to my players primarily as the setting I knew best. That's not the best generic pitch you could give.
The following pitch is a bit unformed but it has positives of the setting in no particular order.
Mystara has a large variety of cultures and countries in a small area. Yes, it's a bit daft, but it does have benefits. Players can be from a wide variety of locales and still interact with each other. It doesn't take for ever to get to those different settings. Adventures can take a different tone or style by moving 200 miles in any direction.
Most of the human nations have analogs to Earth cultures. There are many benefits for players because of this. It allows new players (especially young players) to easily grasp the culture of the nation in a very short description. Fantasy Slavs, Fantasy Byzantines, Fantasy Arabs, Fantasy Vikings, Fantasy Mongols, Fantasy Native Americans, Fantasy British Renaissance, Fantasy Chinese, Fantasy Germans, etc.
Names have meaning. I find the names in FR goofy. They aren't evocative of anything and I'd be hard-pressed to tell you anything useful about a human because of his name (the race of a demihuman can usually be deduced by his name). In Mystara, on the other hand, I can probably tell your ethnicity by your name. Sven - Northern Reaches, Hu Bai Li - Ochalean, Dimitris - Thyatian/Karameikan, Faris - Ylari, etc. Also, this makes the obviously fantasy names of Alphatia stick out (which makes sense as the Alphatians are from another planet).
PCs and DMs can easily incorporate culture into the game. In a game, on of my players decided to be an Ylari Paladin. In his day job, he's an Arabic linguist. Without having to read anything about Ylaruam (which he did, but it wasn't necessary) he was able to do a proper accent and could describe his PC's dress, demeanor, religion, and country with a fair bit of accuracy. Give the player a little information on the country and you have a perfect Ylari PC.
The cultures and nations don't all like each other. There is real bigotry and animus. Sure, Glantri is a completely crazy place and incredibly fun with its French werewolves and Flamenco elves but just try and bring a party with a dwarf of cleric there.
The setting can easily incorporate all tiers of play. The published modules take you all around the Known World. Tier I - Karameikos. Tier II - basically everywhere else (the first 11 modules are set everywhere but Karameikos. Tier II/III - fight a big land war. (not enough land wars in D&D). Tier III - domains or other large scale adventures. Tier IV - fight an Immortal.
Immortals, not Gods. Most Immortals were once mortal just like the PCs. And the PCs can become Immortals, too!
The setting's a bit gonzo. The setting didn't always start this way, but Bruce Heard (who was the setting manager for a few years) certainly took it that way. Invisible moon, hollow planet, flying ships, cargo-cult humanoids, Scottish undead, the aforementioned Flamenco elves, flying gnome cities. You can play it serious, you can play it silly but even when you play it silly all the NPCs take the silliness seriously.
No Drizzt. (Okay, I actually like Drizzt). What I mean is that the setting isn't filled with places, people, or events the players already know about. It's a world that can be a genuine mystery full of unknown places to explore.
Fantastic fan support. Because Mystara was frozen in Amber (pun intended!) over 20 years ago, there hasn't been anything official out for that long. Even when the setting was current there were always undescribed places to flesh out. Since the material isn't canon it's easier to ignore. On the other hand, most of it is incredibly well written so you want to use it.
Alternative History D&D. Mystara is about as old as D&D settings get with the barest introduction coming in X1 Isle of Dread from 1980. It's not an intentionally different place in the way Darksun or Dragonlance is. It incorporated the B/X races/classes exactly as written. Yes, it added many things later but you can look at Mystara as a Parallel Universe D&D (which, I guess, BECMI was to AD&D). Depending on how you incorporate 5e into your game, you can explore a world with no drow, half elves, or half orcs. Explore a world of race-as-class (depending on how/if you use that). For example, in my game all elves must be 1/3 casters and dwarves/halflings can't be arcane spellcasters.
Huh, that turned into something longer than I thought.
TL;DR: Mystara is likely to be unknown to your players but simultaneously easily accessible.
