They've also had Trump supporters come and counter the rally, often with a lot more people.Part of this is by design. The Biden Campaign could likely draw a large crowd if they really wanted to, but just think of the type of folks that would show up. The normal democrat voter is odd enough on their own, but what if some of the antifa types show up?
They don't want middle America to see what a collection of freaks the party really is and they certainly could not control the crowd if things turned violent.