Demas, of live from thessalonica, asks the doleful "What happened?" question with respect to the Universalists. He intimates a too-close relationship to the Unitarians is the problem but the Universalists had enough institutional problems (ministerial shortages were chronic, for instance) and an inflated sense of self (propagated today in outrageous estimates of Universalist strength) long before the engagement with the Unitarians.
A lot of small denominations "failed" because their ethnic base assimilated. See the various Lutheran bodies in the United States. But really they merged and cross-pollinated. Would the Suomi Synod have survived at all -- as the kids stopped speaking Finnish -- had it not folded into larger Lutheran bodies?
For a long time, some people were content to say the Universalists "won" because the doctrine of universal salvation spread. In fact, there were believers in the doctrine long before the first Universalist church or convention formed and there will be long after the last vestiges of Universalist Christianity die out of, or leave, the UUA.
That has to be accepted, but not so grimly, considering the faith that trusts God to be All in All, and in time, to have communion with all souls.