I think part of the reason Epicureanism remained unchanged is because it treated with clear finality certain of its basic premises, and it did so in a way that rendered any further dispute rather pointless among those that accepted the premise. Consider such premises as there is no life after death, and Nature was not created, but has always existed.
Any school of philosophy proposing that there is life after death has not ended a dispute but begun one; what is the afterlife like? Is it eternal or finite? Is it the same for everyone, or will people experience different afterlives depending on their respective portions of fame, virtue, nobility, or piety? How long does the soul linger in the body, and where does it go after? Is there any hope of return from the afterlife? Can people still living contact those who are dead, and vice versa?
Christianity has shattered into a million tiny fragments over questions like these, but for the Epicurean every one of these points of argument is utterly meaningless. There is no life after death, so there's no point in speculating about what that non-existent 'life' might be like. Such speculations, which are not even of academic interest, will certainly never have the power to bring about schism, or mutual recrimination, or factional infighting. And quite a lot of Epicurean philosophy is like that; once you accept the premise that nature was not created by a god, or that the substrate of everything that exists in nature is mere matter, or that the senses are fundamentally reliable, you slam the door shut forever on all of the speculation that does not take its point of departure from that premise.
When we examine the things that did change and develop in ancient Epicureanism, they are quite minor. Epicurus preferred to transmit his ideas in uninterrupted discourse and in plain dress, but that did not prevent Lucretius from casting them in verse, or Lucian from engaging with them in dialectic.