Why is a satellite orbiting a fixed point on the Earth's surface an unbalanced object?

I don't know what you mean by unbalanced. It's no less balanced than any other communications satellite, it just happens to be in an orbit whose period equals that of Earth's rotation. However, some communications satellites including geosynchronous ones can have a m distribution that allows nearly totally pive orientation, using the Earth gravitational field gradient, to point their antennas to the desired part of the surface. This would be done by making the moment of inertia (MI) about the axis that points earthward smaller than the MIs about the other two axes; i.e., like a (mwise) cigar-shaped object. So from that standpoint, it might be considered less "balanced" than a satellite that may want to point stably in various directions and has a more equal MI distribution among the axes.