This is a good manga, however what drags it down is the 90% focus on climbing.
This puts it in a low niche where few people would really enjoy it, those people being climbers. I can understand a manga like this since i can compare its sports density to similar mangas with other sports. But if you're not really into, say, martial arts like we have with Gekiryuuchi, you'll soon get bored of the countless explanations and just skip pages to the story and character development parts. And here lies the problem with Kokou no Hito, these parts are too thin and really not interesting. Like another commenter said, you don't attach to the characters, you don't care. There were a couple exceptions in the beginning of the manga, but soon the author lost interest, killed the characters or just made them disappear with time skips and with no explanation, while skipping further development for the nice views of mountains. I got bored of mountains. It's true i read this in one go, but this way one can see the flaws easier. Really, entire consecutive chapters with views of mountains and delirious ramblings get to you after you've experienced it ten times before.
This is the reason why this manga will never get big, it focuses too much on some parts and too little on others. By the end of it i didn't like any of the characters. I also finished Veritas yesterday and those guys killing each other everywhere were a lot more likeable chars than the ones we have here. Those guys in a fantasy manga had better personalities than everyone here.
One might argue that people in Kokou no Hito are real and that's why they live and act so crappy. But the problem is not with the tragedy in their lives as is with the way they react to it. Let's take two examples:
Yuki gets from being a cute and intelligent high school girl to a stealing whore in just a few years. Plausible, i'm not saying it can't happen, but the chances are low and i, as a reader, need an explanation as to how things got here. I need to understand what i read, not just eat the facts. Also, even in that state she would show a bit of remorse, which does not happen. Returning the money later was just a scheme to get more from Miyamoto. And this is how we get to my second example.
Miyamoto is not that bright and doesn't seem to know anything other than climbing. But what he does know, he does it good - kind of how the mangaka makes a manga all about the thing he knows best, climbing. He's also very ambitious and has actual plans to realize his dreams, he gathers money to get to France. Bam! A couple years pass and Yuki becomes a whore while Miyamoto becomes a failure with no real reason. Yuki tells us he never went to France because that's his nature, a slacker that keeps saying he'll do things "Tomorrow". He got into drinking and debt. How in the hell did these things happen? There's no way a change like this is possible, the Miyamoto from highschool and the drunk Miyamoto have completely different personalities, same with Yuki.
Well, these things make you lose interest in the manga since it's stopped being realistic. As a non-climbing-die-hard-fan you only get left with the pretty pictures.