Higher TTK is more rewarding for players that make mistakes.

That sounds like a bad thing, but there are plenty of games that lose out on their playerbase due to having a low TTK and punishing mistakes harshly.

Call of Duty vs. Halo, Tekken vs. Smash, HOTS vs. LoL, these are all comparisons of similar games that punish mistakes harshly vs. not.

Punishing mistakes is a great way to show off your prowess in that game. They promote competition and are incredibly fun to watch because of how dramatic a single moment might be.

The problem is, 90% of gamers make mistakes and have fun while doing so. League, along with most MOBAs, caters towards the top 10% of gamers, it's unfortunate that HOTS is the best MOBA for the remaining 90%, and it's dying. Still, the fact that it still holds that position while on its deathbed really shows how damn fun it is.