I'd think the Cardassians were designed specifically to be useful general villains for TNG. They had no strings attached, but they had very elaborate and effective makeup jobs and a distinct starship design plus a general backstory of preceding villainy - just like the Romulans originally.
Thus, when the opportunity for a spinoff show emerged, they were a very natural candidate for its villains. TNG already had its own sets of characteristic villains, the Borg, so "donating" the Cardassians for this use was no big loss.
The Bajorans in turn would have been custom-made for the plot of "Ensign Ro", probably with no further plans on their use (or on the reuse of Michelle Forbes' character). But once they were connected with the handy villain species, they were developed further in TNG, and eventually there was enough material there for an essentially Cardassian-driven spinoff, with the Bajorans written in just because that's where random development had taken them in the later years of TNG. I guess the spinoff could have been made without the rather random Bajorans, but it would still have exploited the elaborately formed Cardassians somehow.
Really, the only other likely villain for a spinoff would have been the Borg. Which indeed were rather cleverly written into the DS9 tapestry in the end, that is, at the beginning.
Timo Saloniemi
