For quite a few reasons, although it is worth noting that his ideas were not unprecedented individually, it was only when they were put together that they became something distinctly new. That being said, there were several strands of Greek philosophical/sophistic thought that Epicurus and Epicureanism directly challenged, particularly when all of the elements were put together. Very broadly speaking, the two most important were Epicurus' rejection of the Divine as being present or carrying moral force (when ran contrary to swaths of Greek moral discourse as far back as Hesiod), and his definition of the good life as being one lived for oneself (which opposed civic ideals eg of Plato).

If you want a good idea of the very old Greek ideas Epicurus was challenging I would recommend reading the poems of Xenophanes--we only have a few so it should only take a couple minutes, and it gives a good sense of the reverent and intensely engaged lifestyles espoused by many others.