Because other star systems are so distant, the motion of the Earth around the Sun doesn't cause them to display any apparent motion (called parallax, the apparent motion of objects at different distances when the observer shifts his point of view from side to side). Unless you compared observations made thousands of years apart you would think that the pattern of stars in the sky is perfectly unchanging, i.e. that the stars hold permanent fixed positions relative to each other.

The planets on the other hand, or at least the ones known to the ancients, move very obviously against that fixed background and are quite bright and easy to spot. As they themselves move around the Sun they move through the starry background and when the Earth overtakes planets outside its own orbit they exhibit apparent retrograde movement due to parallax. They also always remain close to a single plane known as the ecliptic, as do the Sun and Moon.

These effects can easily be verified without the use of instrumentation by simply sketching the pattern of 'stars' on successive nights. The actual stars will keep station forever while the planets appear to wander against the background they provide.