Enterprise strongly hinted that they were among the four founding members (there's an episode, "Demons", that involves forming a "Coalition of Planets" that includes Vulcan, Denobula, Rigel, Tellar Prime and Andoria.

As for why the difference of reaction: there are probably two factors. In-story, Michael grew up on Vulcan, so she's probably more affected by its leaving. Out-of-story, the audience is probably more familiar with the Vulcans than with the Andorians.