Sci-Fi by it's nature is going to seem to be less realistic as science marches on, so for a lot of people I think new equals better.
Heavily stylised and not remotely hard Sci-Fi shows like that one are going to escape that sense of outdatedness to some degree I guess.
So between being good, fairly new, and highly stylised I can see how it got to #1.
Plus Sci-fi snobs probably rule out anything that has obvious fantasy tropes and any show with anything in it that seems too much like magic, which probably hurts the score for Star Wars and things like that.
But I think with Sci-Fi more than most genres (except comedy) it probably comes down to personal taste (and what flavour of Sci-Fi you like.)
De gustibus non est disputandum