Why can't flow backward into the atrium when the ventricles contract?

Valves. In a healthy heart there are valves between the ventricles and atria that close when the ventricles contract and stop blood flowing "the wrong way". If the valves are diseased or even missing then blood does flow backwards into the atria during ventricular contractions. That is what is meant by having a "heart murmur". The heart murmur is the sound heard through the stethoscope of blood flowing backwards.