Shows like Andromeda, Mutant X et al survived for as long as they did because they played to their respect audiences, and although they were pitiful series' in themselves, they did that very well.
Also there's budget to consider. Andromeda
was dirt cheap, and that's the reason it's the only non-Star Trek
space opera (except the similarly cast strapped Babylon 5
) to go the distance for five seasons. That's the problem. Without the magic Star Trek
probably wouldn't have lasted for seven seasons, and Enterprise
for four, but both shows were treading both declining goodwill and the continued expense of making the product.
To make a good space opera on Andromeda
's kind of budget you really have to struggle through the mediocrity on hand (the story of Babylon 5
) but to be pretty consistent one needs a good writing staff and good actors, and both cost money.