Really, it speaks volumes about Katsulas that he took a character who only appeared four times (well, really, three. His cameo in All Good Things was just for laughs) and established such a sinister and memorable presence.
If I recall correctly also, the cast and writers weren't too crazy about the Romulans anyway, at least in terms of major recurring villains. They did in a pinch, but certainly not to the degree of development as, say, the Klingons or Vulcans.