Perhaps they wanted you to believe that they built him from assembled parts of other humans, as in the book. Giving an actor's name would shatter that illusion.

"In the opening credits, Karloff is unbilled, with only a question mark being used in place of his name. This is a nod to the first stage adaptation when the monster was billed only as a question mark, and that Universal had not told who was playing the monster, and had not released any pictures of the monster in order to conceal his appearance. Karloff's name is revealed in the closing credits, which otherwise duplicate the credits from the opening under the principle that "A Good Cast Is Worth Repeating". There was controversy around this point originally, as some part of the management of Universal built up the suspense of who was playing the creature to gather interest in the film as Bela Lugosi was still largely thought to be performing the role of the creature up until the time of the film's release. Some papers were erroneously still listing Lugosi as the performer. Some were coming to see if Lugosi had changed his mind and recanted to star in the film despite some published statements to the contrary, most notably the still famous "electric beam eyes" poster which still credited Lugosi as the monster and showed the creature without the now famous flat head, neck-bolt makeup (created by Universal Studios make-up artist Jack Pierce. Pierce also created Lon Chaney's Wolf Man make-up and Karloff's Mummy make-up as well). Others state it was because the film would cause the ruin of the performer in the role and wanted to minimize said actor's liability, for the original film went against the censor boards of the day, which resulted in some portions of the film starring actor as the monster being removed from the film, the most noted removal was the drowning scene of the little girl, Maria. These removed scenes have since been restored to the film releases as shown in the recent DVD releases of the original Universal films." Source and further information: http://en.wikipedia.org/wiki/Frankenstein_%281931_film%29 Further information: http://en.wikipedia.org/wiki/Boris_Karloff