They might be testing the waters for male villains with Lex and Darkseid for Superman and the Justice League respectively. Joker has a universal demand for collectors, comics fans or not. Darkseid is where the crossover begins to dissipate, which is the same argument that could be made for Mr. Freeze or Martian Manhunter, for example.
That said, I more or less consider Batman a separate, likely more lucrative property from their DC line. So, if Sideshow's going to retain license exclusivity for 1:4 scale, they have less of an excuse for not supporting it more.