Seriously, I don't care at all who you play in what lane as long as you win. If you want to Leona top against a Darius/Jax/Fiora, I don't care at all as long as you're not completely stupid about it.
Can someone explain to me why certain champs that can be completely viable in the right hands are completely forbidden by players to go to certain lanes?
It has to do with objective control. Dragon is an objective earlier than Baron is. Consequently, you want two people in the lane nearest to dragon. It so happens that marksmen tend to have the weakest early game and benefit most from support, so that responsibility falls to them. Top lane, farthest from crucial early objectives, is often just a farm lane and permits slower, tankier fighters. That often leaves the team lacking a strong source of AP damage, so a mage is common in mid-lane.
These aren't requirements but there are strategic considerations beyond just winning one's lane that have helped shape the traditional lane assignments of the meta.
It's because it's unnerving to have someone want to go Leona top or anything that deviates from the meta. Would you like a Morgana going top or a Kha'Zix? Would you like a Gangplank to jungle or an Udyr? Would you like a Varus to go mid or a LeBlanc? Would you like a Zed & Gragas in the bot lane or a Lucian & Thresh? Most people would pick the latter options over the first because they're standard meta.
You see, we know that some champions were made for some roles and what roles they exceed in. Leona excels in the bottom lane. When you want to go top lane Leona, you cannot blame others for being uneasy about it because breaking the meta is a risky thing to do. Sometimes it works, sometimes it does not. People want what works on their team; Leona going bottom lane works. Top lane Leona may not work as well. It's all about consistency.
