My current working idea is either a tiny Craftworld or really big ship with a group of maybe 1000 living Eldar tops just can't handle it anymore. Between dying warriors, refusal to return to combat, and the awfulness of using wraiths they just can't field a practical combat force. First use of D-weapons was the last straw and the Farseer basically said "SOP going forward is run and hide, no fighting. If fighting is absolutely necessary none of those thrice damned D-weapons. "
Dedicated Spiritseers were already something of hated pariahs locally and this essentially made them redundant. Mine did NOT like this, also felt it was disrespectful to the warriors residing in the Infinity Circuit. They put the Spirit Stones of the 20 most warlike into bodies, gave the Farseer a piece if their mind, and left. The Farseer wants to bring them back, by force if necessary, but realizes that it would take resources they just cant spare. So he lets the common theory circulate unopposed that he wasn't happy but felt it wasn't his place to stop the Spiritseer / saw some good coming from this.
As for why they don't try to get help from another Craftworld or fleet, they are. The direct path would be a running battle so it's going to be a long, slow, circuitous journey to even arrive mostly intact for either group and it would be risky for the closest Craftworld to try and send appreciable assistance directly to them.