It's quite sad in a way, because Epicurus and the Stoics had quite a lot in common. Both recommended the virtues of a simple life and taught the wisdom of restraining excessive desire and avoiding momentary pleasures.
But while the Stoics stressed that the proper approach to deal with lifes' less pleasant moments was enduring physical discomfort with a calm mind, Epicurus recommended active avoidance of painful or unpleasant things. Not the sort of path you want your legionaries following!
There was also the issue of God, soul and the afterlife, all of which the Stoics took as seriously as Christians, while to Epicurus they were irrelevant or non-existent. Some believe to this day that Epicurus was an atheist. which was not impossible, given that many Greek philosophers of the time had moved past the idea of their Gods as actual beings, reducing them to metaphors or symbols for various ideas and concepts regarding human nature.
Clearly, if you are a growing Empire, you want philosophies and religions that favour discipline and a disregard for the hardships of military life, or a warlike attitude. Which is why the Roman leaders gave preference to Stoicism and Mithraism, rather than Epicureanism and Christianity.
Of course, the Stoics take much of their inspiration from the Cynics, who were extreme ascetics.