I mean, it was the finale of Sonic X, and there wasn't really much chance her character would appear again. Plus, Cosmo was huge for Tails' character growth, centralizing him as no longer just a kid but a hero who puts the needs of the world above his own. I wish the games gave the level of depth to the characters that Sonic X did (specifically in the Japaneese dub. I'm not even going to go into how 4kids messed up this masterpiece).