I have about twenty friends that I regularly do things with. But only 3 of us organize 90% of our activities. A new person is a burden on that core group. We try, but some folks make it hard to bring them in. If they don't rsvp, or use nonstandard communication methods it makes it hard. They don't have to reach out, but they have to respond or after a while we quit trying.

Well, there's a little more to it than that. First off, there's the diffusion of responsibility. The more people there are that can make an invite, the more people assume somebody else would do it. Then there's the group dynamic factor. If anybody in the group has any issue with you, other people can feel awkward about inviting you. The last is the benefit factor. The benefit of getting more ingrained to the community for somebody who isn't established is for the person who isn't established. The people already ingrained have their routine, so their incentive to keep making overtures isn't nearly as strong as your desire for those overtures.

Basic economics (supply and demand) explain a lot of phenomenons: you need them more than they need you.