Because Cosmo sacrificed herself to defeat Dark Oak and the Meterex in the season 3 episode 78: So Long, Sonic, and Tails was very depressed over her sacrifice that he would never see her again.
We the # 1 fans of Sonic X and all Sonic game series believe that Cosmo the Seedrian should come back for season 4 and new game series, because the seed that Sonic gave Tails in season 3 episode 78: So Long, Sonic, is really Cosmo, and if Tails puts that seed in a plant pot and waters it, Cosmo will return from it as soon as possible. So when do we want Cosmo the Seedrian to return in season 4 episode 79: The Return of Cosmo? 2015! Sign this petition if you want her to return.