fit better in already existing structures?
I would say that is the key. I think Stoicism is actually the majority view of most "establishments" in the corporate and governmental and academic world, even if they don't admit it. The prevailing worldview in my eyes is some version of "virtue ethics" in which most existing institutions have their view of what is "good" and seek to apply that to everyone, and that is highly consistent with the Stoic worldview. Stoicism doesn't require overt belief in a particular theology, but it serves much the same function as traditional religion, so it's easy to move back and forth between the two, even if one considers oneself secular / humanist, and still be in the same general area.
Epicurean philosophy is much more 'revolutionary' and "anti-establishment" in rejecting even the possibility of uniform rules of conduct for everyone, other than by agreement, and the idea of placing "pleasure" at the core of how life is to be lived is still frowned upon by almost every other camp.
And also in the mix is that Epicurean philosophy really doesn't lend itself to a hierarchical tightly-organized framework that is conducive to money-making or power, and that in itself is a huge incentive for people who are after one or the other to focus on Stoicism rather than Epicurus.
No doubt there are lots of other factors too but those stand out in my mind.
I think the hurdle that Epicureans failed to cross in the ancient world, and that has to be crossed today, is that if it is every going to thrive as a substantial force it has to find a way to translate the emphasis on "Friendship" into the realization that the world is a dangerous place and that it is necessary for people of similar perspectives to band together in order to survive. The core philosophical elements of that are present, especially in the last ten PDs. Hopefully the internet age will allow that need to finally come together to reality.