Because that's not what it is. It's a 20 minute time penality. And nothing else.
(Also I don't think the 20 minute timeout has any real impact - it's enough for me to watch another episode of anime. 10-15 minutes would be way more effective, 20 mins is exactly the time when you can do something else while you wait.)
You're right, they should increase it to 30 minutes or even 40.
Because having to wait an increased amount of time to play is a reward, not a penalty.
You should also be limited to only 5 random champions while in the "Time Penalty Queue".
It would give me just enough champions to use while I wait.
Well, technically it really is low priority. It's kinda like someone leaving a zombie resistance group to go alone then they call back and say they need help after going off on your leader. They're considered low priority, and most likely won't get help for a while.
I do technically believe that if only 10 people qued up for a game and one had a low priority que that they would get into the game before the time ended. But since so many people play the game in real life application of the low priority que is a 20 min wait.
So you have 2h of free time of your day. Now you can only play one game instead of 2. hence low priority.
If you are in school it may not seem like a big impact to you but with a full time job you sorta feel it.
I'd like a different name for it though. The name "low priority" has been bugging me but I guess it's fine for now :) it doesnt actually affect gameplay
