because button.php can be server intensive and some hosts (usually ones with thousands of sites per server like your host) disable all instances of button.php so your buttons wont work and most of your stats wont track.
Reverse IP for your domain: 1,530 other sites hosted on this server.
From the aardvark topsites FAQ:
Why do my buttons show up as broken images?
There are two possibilities here.
1. Your settings are somehow incorrect
If you have modified the button settings in the admin, you may have broken something. To try and diagnose the problem, right click on the broken image and go to "View Image" to see what URL the button is redirecting to.
2. Your host is mean
Some hosts (actually, just Hostgator, but there may be others) have decided to ban topsites lists. There is nothing you can do about it other than switching to a different host.