It would be the cost. Outlay for sets, etc would be just as great as for a whole series, but for far less gain (i.e. advertising revenue for 2-3 movies rather than for 24 episodes). You could not use stock effects, and the effects that you paid for would be useless after the movies.
The NuBSG mini-series was always intended as a back-door pilot hence the expense of that. The Stargate TV movies already had the Cheyenne Mountain and Gate-Room sets in place, so their initial outlay was far less.
Caprica, the last major 'space' show was seen as a costly failure, and all the major current sci-fi offerings are Earth-bound (i.e. cheap).