Hi, I haven't posted in a while and I gotta first say I like the older, cooler, color scheme better.
That said, I think the Cardassians took over because they were a) interesting to look at, b) their society much still a mystery, and c) they more contemporary villains. The Tolkien elves that are the Vulcan/Romulans had their day but now we've got this skeleton/lizard people to consider. Their society is new and unclear to us - except that they seem more contemporary villains without blades or cloaks or Roman forums from which to declare stentorian decrees. These guys were Nazis with a reptilian mechanized and 20th century flavor rather than an arch flavor of the ancient past.
That said, I think the Cardassians took over because they were a) interesting to look at, b) their society much still a mystery, and c) they more contemporary villains. The Tolkien elves that are the Vulcan/Romulans had their day but now we've got this skeleton/lizard people to consider. Their society is new and unclear to us - except that they seem more contemporary villains without blades or cloaks or Roman forums from which to declare stentorian decrees. These guys were Nazis with a reptilian mechanized and 20th century flavor rather than an arch flavor of the ancient past.