Art is often controversial because

 Well, art is often controversial, because it is sometimes based on the religious beliefs or also the cultural circumstances of certain people...
 ~Meaning, someone may draw something about jesus, and people may consider that controversial... Because they may not believe in that god, or other reasons that involve the religious beliefs of people. 
~Hope this helped! :)

Photography has changed its subject matter quite a lot over the past few years. in the 1800's, it was considered normal for families to have  post mortem photos of their loved ones, and now that would be completely unacceptable. i think that photographers have always pushed boundaries and have been eager to capture everything, it is just society that changes their boundaries.