Something like 95% of professional DotA 2 games end with a concession, whereas I have personally never seen a professional LoL team concede even once, no matter how hopeless the game (though I hear it happens in LCK). Is this a difference of mentality? Does Riot not allow concessions? What gives?
There have been concessions in professional LoL. The most notable example I can think of is at IEM Katowice in 2016, Fnatic conceded at 22 minutes to SKT T1 during game 3 of a best-of-5. It generally happens at tournaments where there’s a massive skill difference and the losing team concedes out of fatigue. In the case of MSI, Fnatic was on their 4th Bo5 in two days and knew they were hugely outmatched so they conceded instead of drawing out an exhausting, losing game.
Mentality plays in part. It’s seen as poor sportsmanship by some, most fans want to see it play out and hope for a huge upset rather than a team conceding out of poor morale. Comebacks and backdoors are frequent enough in League that teams and fans will still feel like there’s at least some chance and try to play it out. C9 vs Samsung Blue at 2014 Worlds is a good example of this. C9 was clearly outmatched but they played out the entire series in hopes of getting an upset.
I’ve only played a few games of Dota in my life so I might be wrong here, but it seems that games end much faster in League than Dota. Cracking a base and killing the Nexus is much easier in League. In the few games of Dota I played, it took a long time(5-10 minutes) to crack the base and get onto the Ancient/Nexus/Immortal. In League, teams can break into a base in just seconds after a wipe if enough survive the teamfight. So it makes sense that if you’re being sieged in your own base in Dota, you’d concede instead of drawing out a long closure, while in League it’s so fast you might as well play it out since you won’t save much time. This is all pure conjecture though, I’m not that knowledgeable about pro Dota.
Another point thanks to @Cronax: comebacks are FAR more likely. In the late-game, even if the enemy is beating on your Nexus, if your team can somehow manage to win the teamfight and kill them all, this can give your team enough time to kill the enemy nexus instead.