2. It is a competitive mode and focuses on the competitive aspects of the game.
It's a competitive mode... Erm... Ok? So what? A really weird reason that has very little to do with disabling it for randoms.
This point imho has a lot to do with allowing only 5-man teams to join it. I will say ahead that I would like to be able to join it solo, but i wholeheartedly understand why it's 5-man only. If it werent 5-man only, you would get more of the people who just play it for fun and just wont put in the effort to win. Odds are you get that 1 mid hero that says "i'm done" right after giving away fb, or you get people who don't ward or, dare I say it, counterward and start calling people who do as tryhards. Making this 5v5 only pretty much forces each team to play tryhard, and a lot of people who would otherwise join it just aren't able to play like that.
Aside from that, the mode seems to encourage thinking of your team composition ahead of time, and you pretty much can't do that with randoms as you won't know their hero role strenghts an weaknesses. Granted, this happens in other modes as well, but as this is labelled as "competitive mode", neither side will really enjoy a game where 1 team has great composition and the other team is composed of 5 melee hardcarrys. The overall match quality drops like that.