Mages are not weak power-wise. They still deal a lot of damage. The problem is that they don't have a lot of agency over the game.
What you need to win in soloQ is proactivity. Being pro active and making plays from the beginning of the game, be that through jungle pressure, lane priority, roaming, solo kills etc. That's how you create advantageous situations for yourself and can then proceed to help out your team, so, if they are struggling, you can pull the game back.
With mages like Viktor, who I mained in season 6, you don't really have that. Your early laning phase is good, but apart from that, you don't have anything going on. Viktor is strong until level 3, and then he has mana issues until he gets his first upgrade, his level 6 isn't that strong compared to others, he can't 1v1 anyone early, he can't skirmish early, he can't roam to help out a struggling team. He can only sit mid, spam waveclear to deny the enemy laner and try to stall up to the point where he has 3-4 items (25-30 minutes) where he can really start doing some work. The thing is, by that point, the game will probably already be decided.
Let me put this into perspective. Assume you are someone like Zed mid and your bottom lane is struggling, maybe they are behind a couple kills, some cs etc. At level 6-7, you are offered the opportunity to shove a wave and then roam bot. Your ultimate is a pretty good tool to secure at least one kill, considering that your bottom lane will assist. This can put a stall on the enemy bot lane snowball, and slightly bring your bot lane back into play.
Let's now assume I'm Viktor, and my bot lane is struggling. Sure, I can shove in the wave and look for a roam bot, but I don't have as much mobility as Zed, but more importantly, I don't have the damage needed to 100% secure a kill. Sure, I can relieve some pressure, maybe they burn a flash or something, but still, nothing of value was granted.
At the same time, I might probably miss some cs in my lane, which is crucial. An early - mid game champion like Zed can afford to lose out on some cs, if it means a roam will be successful and the team gets advantages elsewhere. A late game champion like Viktor can't afford that. They need all the cs they can get so that they reach their late game status asap.
There are a lot of drawbacks that make mages kinda unviable right now. The game pace is so fast, that only a handful of niche mages can be played and that's because they offer other things as well, like Taliyah.