...the attendees may not understand completely that no matter which site wins, voting gives you a supporting membership....

I think they mostly know. I also think that they don't care. They don't perceive any value in buying a membership of any sort to a convention they couldn't conceive of ever attending. Voting rights and publications have zero value to them; they would never vote in the Hugo Awards, anyway. So they won't pay.

has there been any thought to eliminating the fee to vote and allow all paid members to vote? I know that limits the upfront cash for the winning site but possibly there are other ways to make that up.

You sometimes hear this, but there's a sound constitutional reason for it. Remember that it isn't really a voting fee. It's an "Advance Supporting Membership." You're buying a supporting membership in the Now+2 Worldcon, and in consideration of that purchase, you get the right to participate in the process of choosing where to hold that Worldcon. From a constitutional point of view, we don't want people voting who aren't willing to consider WSFS as a whole when they make their decision. That's why we also put an 800 km/500 mi exclusion zone around the current Worldcon -- this reduces the effect of "I'll vote for it because it's close, not because it's any good."

And as far as eliminating a Worldcon's initial cash -- well, I suppose they could immediately start charging even more money for memberships, but that just exacerbates the problem of driving people away due to cost, I think.