HUMS also need to "learn" it's limits of operation. Across a fleet or type, data is gathered and compared across the monitored parameters and as the flying hours build, a set of maximum acceptable limits starts to emerge and these become the "baseline" that is used to decide if a particular component is in or out of limits.
If as stated earlier, the tail fracture was a consequence of a previous incident and/or delamination, it is possible the HUMS would not have registered at all.