There was the SDKFZ 138/1 Ausf H Grille that used the Panzer 38(t) tank chassis (the Ausf K used the front/ mid engine 138 Ausf M chassis, same for the flakpanzer).
Of course they could do a 138 Ausf M, 138/1 Ausf K Grille, flakpanzer and APC kit, but would it have the necessary sales?
If they had not made such a hash of the box text etc, they could have done a real 138 Ausf H kit as well as the 139.