It's probably because the Clans represent the greater threat and therefor present the greatest challenge. If the game is successful, there's no reason why they can't make an expansion from a Clan's perspective. I suspect that even if it is Inner Sphere, you'll eventually be able to pilot a Clan mech. And with all the infighting, you'll probably get to shoot at Inner Sphere mechs on occasion, so all mechs will eventually appear in your sights.
But to be honest, they've been telling us to "wait about a month" for a while now.