Thor chose to neglect the silver pine tree in favor of other things that could improve his strength directly, such as his golems, which he calls his sons.
But the higher-ups and the patriarchs continued to value the silver pine tree and were paying attention to anything related to it. So Arthur's mission drew the attention of the patriarchs of the Blood Skull sect.
The patriarchs considered shutting down the mission briefly to maintain secrecy and control of the item. But they didn't do that because they didn't need to shut down the mission to keep the secret of the silver pine tree from leaking from the sect.
Everyone in the sect had already sworn an oath not to share sect secrets. All the sect needed to do was deem the information about the silver pine tree a sect secret, so anyone who wanted to speak to outsiders about them would be unable to.
