A natural immune response will contain antibodies that will attack all parts of the virus, putting selection pressure towards changing the shell of the virus wh

A natural immune response will contain antibodies that will attack all parts of the virus, putting selection pressure towards changing the shell of the virus which can be done without compromising the functionality. By only providing the spike protein via a vaccine, you ensure that the selection pressure is against the key functionality of the virus. Changes to the spike sufficient to evade the antibodies would also reduce the effectiveness of the virus.