The wide proliferation of really terrible ones seems to tie into some murky SEO tactic of getting a designer to whip you up something 'viral', stick it on the www.totallylegitadultdiplomaswithfreeviagra.com root, and then have a cute little 'share this' link that gives you the url along with a blob of html stuffed with invisible SEOjunk keywords, and rely on people's laziness and stupidity to get the backlinks flowing.

I recall reading a good article on it a while back, but oddly enough, "SEO infographics" is now a sufficiently desirable niche that there are junk infographics just waiting to take your call.