As other users has mentioned the line between science-fiction and fantasy is often a very grey, blurry one. With mental based sciences and technology based magic the two genres can, and do, frequently cross.
As to the question, "Why is non-science fiction based fantasy part dominating?", I do not readily agree with this belief. It is true that strange symbols and flying, angel-esque creatures may predominately feature in fantasy, but this does not preclude their use in, or their application to a science-fiction based motif.
So then one might ask, why not add something more readily recognizable as science-fiction to the graphic (ex. retro style science-fiction rockets, robots, ray guns etc)? This is a valid question. In my opinion the graphic adequately represents both science-fiction and fantasy as it is (in the broad, general definitions for both). Beyond which, if more hard science-fiction themes are added to the graphic then people who have a predilection for fantasy may begin to feel that the graphic is too tilted toward science-fiction. They will then ask to have more traditional fantasy themes included (wands, unicorns, dragons etc). This sets the stage for a never ending back-and-forth struggle.
As it is now, the graphic keeps it simple. It requires the user to use their imagination a little and make it what they want it to be (as any good science-fiction or fantasy story should). It strikes a difficult balance between two worlds that would be all too easy to tip to one side or the other. At this time I feel that a redesign is not necessary and that as it is now lends itself to the design of the site very well.