Some people, me included, dont have the time or care enough to write explanations why and honestly it doesnt matter. When you limit the amount of characters anyone can vote for, it forces them to choose wisely and also makes them votes on the ones they are most sure about being in the wrong place.
If people were voting on literally the entire cast they are far more likely to move characters without any reason.
As for people voting for their mains, as others have said, in fact it is the people who use the characters who have a better understanding of them.
Think about it, should I ask sheik mains who have never even played as Bowser Jr, how viable Bowser Jr truly is or should I ask people that actually use him in tournament and have experienced all sorts of matchups as him?
For the record, I think Roy is better than Marth, and Lucina is better than Marth. Saying that people dont know what they are talking about by suggesting Roy is better than Marth, suggests that you know for a fact he isnt. But is there proof? No one really knows for sure since they are both so underplayed. The only tournament videos I ever see with marth have him getting bopped, and his mains all dropping him for other characters.