Well I didn't say that's the only reason, but it's a major reason. Anyone can try it and win, but once the game starts getting harder (which is not a problem for experienced players) it's not so hard to make you stop playing and you have plenty of choice to experiment with your build. When that starts not helping as well you start going to forums. PoE completely misses the second step (experimentation) when it comes to new players because of its built in restrictions. You have to start anew or farm a lot in order to try even a single alternative.
We didn't need to be coddled because most of us started with the simplest games like Super Mario and while the gaming industry evolved we evolved with it. I myself started on the Atari. Trust me, I didn't need guides or tutorials to know to press a single button and tilt the joystick right to jump over a puddle. That encompassed the entire game.
Some players that have joined after, say, the year 2000 might not immediately find some games that actually navigate them into what to do and how to be successful at said game (apart from actions or arcades). Some of them have struggled and are now successful normal gamers, some didn't and think games are a waste of time and/or not worth it. And now we're past 2010 and people are actually angry that some games might actually offer a learning curve instead of being presented in a "stair type" curve where you just have to figure it out before you can start enjoying it, then fail and start learning anew to again start enjoying it (possibly even more).
Just because you've played RPGs for the last 15 years and know the genre inside out doesn't mean new players will have the time and/or patience to devour every detail out there. When people like you and me enter a new RPG everything is immediately familiar - health orb/resource orb, skill slots, skill trees, every type of potion imaginable, mob diversity and how to handle them, stat sheets, item stats etc, etc. You really think it's bad to actually let players be introduced to these principles of the genre before you start making them take decisions on things that would last them for tens or hundreds of hours?
And when you say "Every player in PoE is a «new player»." it's just so wrong. We are the old gamers. We're pretty much built to do this. The mechanics I described earlier are engraved into our brains. That goes for almost all people that went to PoE because it's harder to play. Because they're more dedicated than new players, because new players don't even know the basics of RPG games. And Diablo is a perfect game for new players. You can progress plenty without trying to figure out too much, but to be successful at the end you'll have to start mastering the game.
Before someone comes in and says "no, you can master the game without even using your brain"... just don't use this kind of trolling. You know very well that's wrong. And please accept that I am indeed agreeing PoE is harder. That is its primary feature after all.