Because, as a society, we are nearly numb to murder. Rape is something that we consider barbaric and wrong.
The argument that murder is worse or equally bad is a bad argument, because even if something is bad, it does not mean that adding something else bad makes anything better.
Bottom line is that the Elder Scrolls, and Bethesda in general is not a company who are known to dabble in taboo. If say, Rockstar, were to add rape, the responce would not be a negative. However, even if they did, they, like anyone, would catch to heat of many feminist groups.
Blood and Gore is banned in many countries, like Germany and I believe, Australia. If rape were to be added, those countries, and many other governments would start laws and regulations preventing rape from being shown in media.
If you want blood and gore, you can wait for a horror game, buy a horror game, play a different game, download a mod, or go lobby for fluffing rape on an internet forum.