Browsing the Gretsch Guitars website, it seems that all of the guitars with Dynasonics utilize the tone pot wiring...while the Filtertron equipped guitars often use the tone switch wiring. (meant as a "general statement", not meant to mean "no exceptions" cause I really don't know!)

My question: Is there a reason why dynasonic pickups are not in the current line up wired with a tone switch ?

Because historically Gretsch didn't introduce the tone switch until they introduced Filtertrons.