I think the answer has to be "Because there isn't any modern sci-fi"
Under Sci-Fi or Fantasy there are plenty of modern shows. And most aren't humourless and even BSG, the biggest example, had its own dark humour there.
I mean off the top of my head the SF&F shows that are currently on...
The Almighty Johnsons
Once Upon A Time
Game of Thrones
Yes, most of them are more Fantasy than Sci-fi but most of them are also far from humourless.