The way is was 'splained to me is like this:

To optimize the sound of a guitar's top, it needs to be put under additional tension above and beyond that provided by its construction. On a flat-top guitar, that additional tension is provided by the strings' literally trying to pull the top off the guitar. Instead of lying flat and relaxed, the top is "tightened" by the string tension.

On an archtop guitar with a floating bridge, the additional tension is provided by the downward pressure of the strings on the arched top.

As a general rule, a floating bridge on a flat-top guitar is going to produce a dead sounding top because downward pressure won't activate a flat top unless some ingenious structural modifications are made.