Not the reason, you cant go bot because of the AD. You cant go bot because you will constantly harassed for a melee minion. The reason I would say Darius is a bad pick because of his mana. Also he easy to kite. Even with a blitz a more aggressive support, I would feel at home. The moment that skillshot misses, punishment is coming. However if he doesn't miss that is also fine. I didn't dodge it. Melee bot is more a cheese lane. It's a gamble, that many people do not want to take in ranked. Most of the time it will fail on them. That is also why they dont take that gamble of the cheese lane. The cheese lane gets behind? It's going to be a horrible existence for them.
Though is they do good its good for them. But it's not a lost cause for the marksman and support. All they need to do is farm up, and slowly gather power while trying to get ganks from the jungler. He will come since its bot lane.
Though that is another reason why people dont want to play the melee bot lane. Make or break. They dont want to play a make or break lane. Cait vs jinx? Cait wins, however Jinx can also win the lane. Once she is winning that lane boom freedom from the oppressor. Cait, Draven, and Lucian are the biggest bullies on bot lane. Put a melee against them and they almost always lose, even with a blitz or more aggressive supports.
They dont want to play anything besides ADC because that is all they know as well. Also they dont want to figure out what melees are good against a ranged champion. However that doesn't mean they are bad or anything. They just dont want to do it because if they get behind a ranged as a melee it's going to be even worse than a ADC is behind. Also the ADC is in constantly doing damage in a team fight. Why because he is Ranged. Why are high damaged players always ranged? Besides they have distance from the fight. The highest damage players are never in front of the boss, because they have such low resistances. It's fucking stupid. That ADC could possible get deleted in one hit. Why would they ever do something so stupid?