Why Do We Have Lightweight Torpedoes?

During the Cold War, the US Navy concluded that lightweight torpedoes were largely ineffective against Soviet submarines, in terms of achieving a kill.  Even heavyweight torpedoes were questionable and thought to require multiple hits to kill.  Lightweight torpedoes were considered more of an annoyance to Soviet subs than a lethal threat.
 
Lightweight torpedoes are a lethal threat only to small, lighter built diesel subs.
 
Lightweight torpedoes are not a ship-sinking threat to anything much bigger than a patrol boat and are not capable of sinking surface ships, either merchant or naval.
 
That being the case, why is the Mk54 lightweight torpedo so ubiquitous throughout the surface Navy?  Wouldn’t it make more sense for ships to mount heavyweight torpedoes?
 
Here’s a brief comparison of the Mk48 heavyweight and Mk54 lightweight torpedo characteristics.  Characteristics vary, of course, depending on the exact model but these are representative.  Note the lightweight torpedo’s warhead weight and range deficiencies compared to the heavyweight torpedo.

The Mk48 heavyweight torpedo can do anything the lightweight can (which isn’t much) plus it is capable of : 

• Sinking merchant ships
• Sinking submarines
• Sinking surface ships
• Destroying maritime structures (oil platforms and such)

 
There have been reports over the years that the Mk48 has problems in shallow water.  The lightweight torpedo was intended, in part, to be the answer to shallow water issues but has had its own reported problems.  The last DOT&E annual report that I can find for the Mk54 assessed it as operationally ineffective (2014 report) and not operationally suitable (2023 report).
 
All of this leads me to ask, why isn’t the Mk48 standard on surface ships?