Why was there never a major reoccurring Romulan character in a Star Trek TV show?
The Humans got plenty of characters.
The Vulcans got Spock, Tuvok and T'Pol.
The Klingons got Worf and later Martok as reoccurring character.
The Bajorans got Kira and to a lesser degree Ensign Ro.
The Ferengi got Quark and his family.
The Cardassians got Gul Dukat and Garak as major reoccurring characters.
Even the Borg got 7 0f 9 and the Borg Queen.
The only reoccurring Romulan characters were Commander Sela and Commander Tomalak. Sela only had two appearances before being forgotten and Tomalak only had two appearances that were important to the story, his third appearance was just a hologram and his fourth appearance was just a cameo.
I think the Romulans would feel less underdeveloped if there was a major reoccurring Romulan character in one of these shows.