What causes multicast traffic to permanently stay on the shared tree and not switch to the source tree?

Setting the SPT threshold to infinity would also cause traffic to stay on the shared tree, but filtering RP announcements is a common misconfiguration that leads to the same outcome due to lack of source information for SPT initiation.

An incorrectly configured RP IP address would likely prevent the shared tree from forming or functioning properly in the first place, rather than just preventing an SPT switch.

If RP announcements are filtered, routers may not receive the necessary information about active sources or the RP's full reachability. This lack of complete source-specific information, often disseminated via the RP, prevents routers from initiating or successfully building a direct Shortest Path Tree (SPT) to the source, causing them to continue using the less efficient shared tree.

Source Specific Multicast (SSM) explicitly uses source trees and does not involve RPs or shared trees, so traffic would not be on a shared tree at all in this scenario.