Five Titans, five seasons, but Starfire got none. Season 5 was her turn but instead it was Beast Boy's second season. Subsequently her archenemy is the only archnemesis to not be main villain of a season (Slade got two seasons, second being Beast Boy's first season).
She's my favorite Titan so that's one of the few things I dislike about it.
So why in the name of Sephiroth did they single her out in this way? Is it because of how powerful she is? I heard someone say writers dislike really powerful characters or something. But Raven's pretty powerful too, the way she destroyed Trigon, who seemed beyond even Starfire, and Raven got her own season.