you can certainly argue it should have been a default warning from the beginning, but mistakes were made and it's a bit too late to change that. people don't like when old, battle-tested code suddenly starts spewing a new warning everywhere after a compiler update.
in reality, most production build systems at least use -Wall (or their compiler's equivalent) and possibly also have a list of specific warnings turned on/off for different parts of the code. it would be nice to have some saner defaults, but it just doesn't matter that much.