It's honestly just stupidity on Riot's part... not a lot of changes have made me as mad as this one. Seriously, "peak hours?" Do they even realize how utterly stupid that sounds? It made me laugh so hard. At first I saw it on Reddit thinking it was a joke, but when I did the digging and found out it was real? I couldn't believe it. Honestly, it was such a joke I wish they would have just left it out of the game.
This especially coming from a gamer like me, who regularly plays multiplayer games, where every playlist is accessible at all times, although, it'd have to be a little more constrained with skill based matchmaking and the fact that you have to find ONE person for each role on League of Legends.
Still, even considering all that, ranked 5's should always be active, not just during "peak hours." Lots of fives teams only do it to play for fun with friends, and they can range from bronze to diamond, and more importantly, it has NO EFFECT on their solo Que rank.
Thing is, you have to get 5 people to WILLINGLY agree to play 5's, going in knowing that there aren't a lot of other teams to play against and they could easily be faced against a bunch of diamonds. Guess what? They don't care. They're trying to get competitive with their friends, try out new startegies in a non-toxic environment, and just overall have FUN, a different kind of fun dynamic que can't give them.