Because it's more challenging. All the other reasons are kind of not true in a way or another.
No RMAH don't matter you can just chose to not use it on softcore.
But if you talking about easying your way by paying for items, it still happens if you want to do so. Buy gold and buy items on gold AH. Same exact concept and outcome.
It's a worse community actually, asside from your group of friends you find hundreds of assholes trying to kill you with elites by joining their game. Not even a public game is needed. I've been invited to trade with someone and almost got killed. And you also have ppl doing trade scams like in softcore.
It is more challenging but not really more exciting it depends on ones opinion. I think zerging things and dying to funny stuff much more exciting than rinse and repeat playing style with alot of restrictions just so you stay alive
Bigger E-peen is very relative, the game is very easy as it is given the right gear and spec. I'd say having the highest dps and best gear is much more E-peen than killing inf diablo
It's not more like d2 at all. Its like d3 softocre with HC mode aka die = done
Die = dead is the reason why it's more challenging. So yea thats basically same idea.
HC is for those who like a challenge, but once u kill all bosses HC is basically over. Not really a point to farm elites. Too dangerous given non game related issues like lag spikes etc. And u dont really have a reason to farm. You can't even try and make some $$. For fun by using the RMAH becuz HC don't haz it

Long story short, chode