The physical Koinonians destroyed their civilization in a series of bitter conflicts, lasting for generations, centuries ago. Those conflict were later called the Koinonian Wars. They left ruins in underground caverns that were studied by an USS Enterprise-D away team in 2366. Markings in those caverns and subterranean tunnels were identified as Koinonian thereby making the identification of the planet as their homeworld possible. One room discovered was a large ceremonial chamber, but it was trapped by the Koinonians with subspace proximity detonators, remnants from the Koinonian Wars. Only sparse data about the corporeal Koinonians survived into the 24th century.
The non-corporeal Koinonians had strong morals and watched the raging wars in abhorrence, but were not involved as the war was fought between different factions of the corporeal species. They survived on the homeworld even after it was ravaged by the wars and still inhabited the desolate planet in the 24th century. They considered the wars a tragic but dishonorable past and didn't tolerate any more suffering on their homeworld. Therefore they created a replica of Marla Aster who died in 2366 on the planet, due to an old forgotten subspace proximity detonator and decided to take care of her son Jeremy Aster after his mother's death. They were able to recreate his parents' former home on Earth just by reading his memories and, in order to recreate the Marla Aster copy, drained antimatter from the antimatter pods of the Enterprise-D. They later accepted that Jeremy was in better hands with other Humans and the Marla Aster replica ceased to exist. (TNG: "The Bonding")