You see them here and there........ often as private-purchase guns.
The Defender 1300 is descended from the 1200, which is a modernized Winchester Model 12, which prior to the introduction of the 870, was THE pump shotgun of the shooting world (yeah, the Remington Model 10 had a lot of fans, but not as many as the Winchester. Even the Ithaca 37 didn't have as many fans as far as I know).
So there's nothing "wrong" with the 1300 (FN's new Police shotgun is essentially a 1300 on steroids). It's just the 870 has the market sewn up.