This seems like a rather large exaggeration. To begin with, Batman's #1 villain isn't as successful in a solo capacity as Lex or even Bizarro. There are Catwoman and Harley but I'm not sure you can still call them Batman's villains any more than you can identify Steel or Supergirl as "Superman's partners."And it would take even more effort to put them on par with Batman villains, whose personality is so strong and interesting and complex that some of them can be the main characters of entire stories and series, too.
Joker is "so crazy, he can do anything!" The rest of the villains, when they try to branch, get turned into anti heroes or stories like One Face. Which of course sold very well. It's easier to be a winner with Batman, because Batman is a winner.