As I recall, when Red was doing his bot scanning and reporting, three ISPs had a third of all the infections.
As was mentioned in the slashdot comments, you need to know what the denominator is -- what percentage of computers use those 50 ISPs? Half the bots isn't impressive if they have half the computers.
Assuming it is disproportionate, it probably reflects the 800 pound gorilla effect. If a small ISP is harboring a spam-spewing computer on a dynamic IP range and doesn't deal with it, it's going to get a reputation for being a rogue company and find its IP range blocklisted. If a company like Verizon is harboring spam-spewing computers, no one is going to blocklist their entire dynamic IP range. They'd take flack for blocking too much legitimate email. (No one except SORBS would, anyway, but you'd be insane to depend on SORBS to filter your email.)