Because THEY have rules regarding certain material, I believe. There are others that work here. I use Imagevenue, for thumbs and links and Fapomatic for full size pictures.
Most mainstream hosting sites prohibit "obscene" material in their TOS. They also don't want their services used on porn sites so they've blacklisted us. Absolutely nothing we can do since it's their end, not ours.
Automate started a pretty good thread about photo hosting companies that are adult friendly: