I'm sure it's the fact that it is best for graphics -- people like their pictures.

Sadly, it's not good for commenting on the images. I'd love to go into an art tumblr and be able to say "I like your art -- I especially liked your use of color/light/line. However, you've put six fingers on your character/given him two shadows/made a male animal have female coloration", but it's impossible.

And I understand that Tumblr is hostile to complaints, ranging from people asking for more accessibility all the way down to people asking for help logging in.