The Torah tells us in Vayikra 18:3 "you shall not follow their rules," referring to the laws under which the peoples who inhabited the land of Canaan lived, before it became the Land of Israel.
Our sages learn from that verse that there is a prohibition against following the laws and customs of non-Jewish peoples. For example, it is forbidden for a Jew to wear clothing specific to any idolatrous practice.
According to some opinions, bringing flowers to a cemetery is part of this prohibition. That is, the fact that non-Jews customarily bring flowers to cemeteries when visiting their deceased relatives makes this custom forbidden for Jews.
Instead, it is customary to place a stone on the gravestone with the left hand, symbolizing that one is doing it reluctantly, against the grain. The idea behind the stone is that it is something through which the deceased knows they were visited.