One major problem was that the show really didn't have a plot. The whole "Lost in Space" thing is good as a general outline, maybe something they could stretch out for 2 seasons but after that you usually need something else to drive the plot.
TOS Plot: Exploring in Space
TNG Plot: Exploring in Space, side of Klingon/Fed politics
DS9 Plot: More plots and plot than you could shake a stick at
VOY Plot: Lost in Space, side of Borg politics
ENT Plot: Newbies in Space, one season of War Plot