Prior to the invention of fly by wire technology and computer driven FCS software, it was problematic to design a wing that would give a good compromise between docile low speed handling, maneuverability and low drag at high speeds without the aircraft being too aerodynamically unstable to actually fly in a controlled fashion. Swing wing was onesolution to this - but the rise of FBW meant aerodynamically unstable configured aircraft's handling issues could now be tamed, rendering swing wings as un-necessary.
Fly by wire uses computers to potentially make hundreds of adjustments to the aircrafts control surfaces every second without the pilots input, in order to ensure the aircraft remains in relatively stable flight.
That a swing wing configuration often results in a lot of additional weight, and often too a maintainance intensive airframe, is another reason its been abandoned and no new jet design since the early 1980s has seen it used.