Presently I think most PTB (the decision makers) in tv just don't have the imagination for a decent space adventure idea. Hell it isn't even on their radar because they're convinced it's too expensive for something they likely don't really understand anyway.
TV is a business. The shareholders expect maximum returns for minimal investment.
Scripted drama is hugely expensive. Scripted comedy is even riskier; it needs highly-skilled writers and actors, or the show just isn't funny. Scripted SF drama has the additional costs of SPFX and unable to use existing locales and sets. All drama is very risky TV, money-wise.
Actors are very reluctant to lock themselves into five year minimum contracts in case they must turn down other roles.
So limited-life lifestyle programs, quiz shows and reality TV shows are comparatively more economical - and these days the audiences donate money to the studios "vote" the players out of reality shows.
Not to mention that so many channels, plus cable, the Internet, X-box, Wii, etc is splintering the old huge, dependable slices of audience. When CGI SPFX is cheap enough, someone will come up with a SF series where cast members can be voted off, and maybe there'll be a new contender for Show of the Year!