Required premade? why?

So I've been playing this star guardian thing for the last while, trying to finish some of the challenges, and after getting a random group that decided to try to get the "Burn Bright: play with janna, lulu, poppy, lux, and jinx" achievement thingy, we finally succeeded!

At the end, nobody gets the achievement, and we re-read the mission text. Why on EARTH would Riot decide to make this premade only..? I mean, why wouldn't you want to encourage people working together, regardless of premade or not? I can't even imagine what would have pushed someone to make this decision.

This is seriously a letdown. I don't have a lot of people on my friends list, because I can't speak (work accident), and most people prefer I use discord or something similar when we play. I find that awkward, because it's 1-way communication. I'm used to this situation, so it doesn't bother me. But please don't put quests in the game that REQUIRE me to HAVE friends on my list, rather than manage to work together in random groups. Despite what some people might think, sometimes random groups are really great, and fun. We should be able to get the achievement so long as we manage to complete the objectives as a team. I would even suggest it's HARDER to do as a random group, and I think that should be rewarded in the same way.

I mean, what could the argument possibly be against letting everyone have a shot at this? I don't get it. Riot making some strange decisions lately.