Is there even a current tv sci-fi series of any real note on right now besides DW? Or even in the last few years? There's not a whole lot of mind-space competition, really. I wonder how good DW would be doing if there were another (or more) sci-fi show of similar quality continuously in the US/Canada since Enterprise ended. Battlestar Galactica took that role for a while, but it's been over for a while.
Also, DW is a geek show, while Trek is a nerd show. That's not to disparage either show, but they have somewhat different, though often overlapping, audiences (see: this thread).