I think part of it is that if you don't pay the big bucks they won't consider you for the front page and just rotate people in. With a larger page and tons of impressions, you get a lot of permutations and probably only miss the front page like 1 out of 100 times. Wedding wire did say their $250/month package was sold out in my area, so there's a limit to it even if just to get you to buy the $400/month package.
I had a $10/month listing on second shooters last year that worked out pretty well. I only had it for about six months or so, and got a $300 second shoot and a normal $250 main photographer/non-wedding gig out it. The guy asked me to second shoot again for $300, but I was already booked that day. I was really surprised so few people paid the $10 month (like 2 or 3 others in my area). Even if the investment paid off, I was surprised I only got two contacts from it. I guess if it's $10 a month then not a lot of people use the site.
Those kinds of listings make sense and I would conceivably pay thousands to be on a site like The Knot but not if I am paying that money to shoot a price points where I'm not making money anyways. I have noticed companies like "The Pros" and "George Street Wedding and Video" prominently featured on Wedding Wire, The Knot, etc. They're studios that employ dozens of wedding photographers for those unfamiliar with them.