"East is that way."
I'm curious on this as well. My best guess is that since そっち means it's closer to the listener and 'east' is more of a directional concept and not an actual pinpointable place. That the listener can't actually be close to "east" and therefore あっち to imply east is a far off place away from both of you is more suitable.
The listener could certainly be farther east than you so socchi could work in the sense of east is 'in your direction relative to myself', but not "east is near where you are" which might be what is more implied?