A schedule move rarely happens because a show performed badly on its own terms. It happens because a broadcast schedule is a system where each hour is meant to support the ones around it.
A grid is built to hand audiences forward
Broadcast programmers assume a portion of the audience stays put between programs. A strong show is therefore placed to lift whatever follows it.
That inherited audience is the most valuable thing a new series can receive, since it removes the need to find viewers from nothing.
Moving a show is often an attempt to spend that inheritance somewhere it will do more good, which necessarily takes it from somewhere else. Every gain in the grid is funded by a loss elsewhere in it.
Counterprogramming targets a rival's weakness
Networks also move shows to attack a competitor. A drama with a distinct audience may be shifted opposite a rival hit that appeals to a different group.
The aim is not to beat the rival but to capture the viewers that rival was never going to serve on that night.
This works better with shows that have a defined and loyal audience than with broad ones, which simply lose out in a direct collision.
Sports and seasonal events force disruption
Live sports and awards broadcasts occupy large blocks of the schedule and shift everything around them. Some series are displaced for weeks at a time.
Long gaps break the habit that keeps viewers returning, so a show can come back to a smaller audience despite nothing changing about the show.
Networks sometimes move a program permanently rather than return it to a slot the audience has stopped associating with it.
The move itself carries a cost
Viewers who watch live build routines. A change in night or hour severs that habit, and a meaningful share never re-locates the program.
Promotion can partly offset this, but promotion competes for the same limited airtime as everything else the network is launching.
The result is that a moved show frequently loses audience regardless of whether the strategic reasoning was sound.
Delayed viewing complicates the whole model
Much of a modern audience watches within days rather than live, and measurement windows account for that. The value of the slot itself has fallen accordingly.
Advertiser deals still price against defined windows, so the schedule retains commercial meaning even as viewing behavior loosens.
That is why the practice persists in an environment where the underlying assumption about audiences sitting still is weaker every year. The grid survives as a commercial unit longer than it survives as a description of behavior.