Greetings!
Wow! Well, hmmm...some thoughts--
Harn, like Rolemaster, is written to a higher reading level--their market niche has generally been college-level adults, whereas D&D--back in the day, as currently in 3rd Edition--is written at a lower reading level, and generally aimed at the 12-18 year old market segment. Obviously, then, as now, many people who are older than that, and impressively educated, play and enjoy D&D.
I happen to like D&D very much! I also like Harn, and I have some Harn stuff in my library, as well as D&D stuff, Warhammer Fantasy Roleplay, Talislanta, Rolemaster, and so on.
Making such an observation shouldn't insult anyone. Those are merely facts, and I have seen over the years where people who worked for Rolemaster and D&D have said just that. I haven't read it expressly from Harn, but from the reading level evidenced in their various rules and modules, it does seem to generally be at a higher level than D&D.
Is the prime age-market for D&D generally teenage males?
Well, yes. Is there any who would seriously deny this fact?
Rolemaster has a primary market focus of college-adults, generally 24-and up. I've played Rolemaster for some ten years, and as a personal observation, I can't say that I have ever met anyone who plays Rolemaster who is younger than 22, and quite educated.
Rolemaster is extremely detailed, with charts and charts and tables for everything. It can, for many, become incredibly complicated, in a number of ways.
For anyone who has played Rolemaster, is there any real doubt about this fact?
I also played Warhammer Fantasy Roleplay for about four-five years as well. Warhammer is generally low-fantasy, low-magic, and very gritty, if with a quirky, dark sense of melodrama and humour.
Still, after awhile, Warhammer has it's limitations, and there are some very cool campaigns and interesting stories that only D&D can do. Warhammer can't do many of the cool things that D&D can do. That's just the way it is. Gritty and "Low-Fantasy" can become old.
Harn seems--in whatever edition--I've seen several over the years, and Harn does go back quite a ways--to be quite detailed, and more complexly designed than D&D. It aims for realism, and accurate details in everything.
Harn tends to remind me a bit of a strange blend of Pendragon, Warhammer Fantasy Roleplay, and Rolemaster.
Still, Harn is detailed, and interesting. Even if it is low-magic and gritty, isn't that ok? There are a whole lot of people here at EN-World who sing the praises of GRR Martin, and grittiness. I'd think that Harn would be a great rave here!
Then again, what's so wrong about D&D? Grittiness and low-fantasy can be great--I happen to mix a lot of grittiness into my own campaigns--but, like "High-Fantasy", it too, can get boring and dull. Then, you need lots of dragons, powerful magic, and crazy monsters running around!

Isn't that ok, too?
I suppose I don't understand all the heat generated about a game system. I play D&D for many reasons, but two of the biggest reasons are the following:
(1) D&D is simple, swift, and smoothly deployed and applied. From skills to spells, to classes and combat, everything works relatively smoothly, and is easily understood, whether I have players who are 12, and just starting to get into roleplaying games--a 20-year campaign veteran, a razor sharp college student--or someone who is a professional adult, who has never gamed before, and is just getting started.
(2) Unlike Rolemaster, Harn, or Warhammer Fantasy Roleplay, it is far easier to find new or additional people who are interested in playing D&D. Attracting new players can, at times, be a distinct challenge for using virtually any game system other than D&D, or perhaps Vampire or whatever White Wolf does. Try Harn, Rolemaster, Pendragon, Gurps, Warhammer Fantasy Roleplay, and so on, to find players--or *gasp*--someone experienced, knowledgeable, and willing--to GM those games! I have found over the years a few players, beyond my tight group, but GM's?--in my experience, they are very unusual.
However, D&D is a big name, easily identified with, and many people are enthusiastic and eager to play 3E D&D, for the first time, or to get back into gaming.
For example, this last week, I got my two nieces, and my mother-in-law to sit in on a game of D&D with my wife and sister-in-law. If I had dropped Harn or Rolemaster on them, they would never have been able to play.
That isn't to say that any of them are dumb, or uneducated, but like many other gamers out there, many people who game are not interested in *large* amounts of details, too much reading, and too much activity that reminds them of work, in order to play. I have one friend, he loves to play, and he's quite intelligent, but he just doesn't have the patience to read for more than about 20 minutes, or a few pages. He gets tired of reading, and wants to get on with it and play. He is more of a "doer" than a "reader."
To be honest, a lot of people who play nowadays--that are adults with jobs, college, or kids--simply don't have the time or energy to read lots of books and absorb lots of detail. I am a DM, and yet, I can say that I have to focus my time very carefully, and balance many different responsibilities. There was a time when I had nothing but time!
I say that as merely some thoughts on why games like Rolemaster, Pendragon, and in this particular case,--Harn--are not anywhere near as popular as D&D. I think these factors play into those games being less attractive, and therefore less played.
Something else--I like lots of details, and even realism. I love details, actually!

However, as noted above, speed, simplicity, efficiency, and the fantastic, can all come in very handy. Realism and detail, taken to great levels, almost inevitably slow the entire game down, and make everything a longer process. That slowness of pace, and longer process, even for educated, intelligent people, can in itself become tedious and vexing. Eventually, like with D&D, everything can feel quite refreshing, and easily changeable. D&D's great flexibility is refreshing, and even inspiring.
For example, with Warhammer, Pendragon, or Harn, you can *only* play *ONE* kind of game--that is generally gritty/realistic/"low-fantasy". However, with D&D, one can mix and match rule variants, tweak details, use or discard options, and play "Low-Fantasy" or "High-Fantasy"--or something in between. And yet, D&D will still work just as smoothly, just as simply, just as flexibly.
One cannot change such with Harn, or Warhammer, or Pendragon, and have it remain the same game. Such is "hard-wired" into the system, as it were. To alter such a fundamental series of concepts would tax the system to its breaking, for they were designed expressly to be "low-fantasy"/gritty/realistic. Changing those base parameters distorts the game beyond recognition, and presents an array of problems.
Well, it's late, so forgive my ranting. I like D&D, and I also like Harn. They are both cool game-systems/worlds. Remember to have fun!
Semper Fidelis,
SHARK