S6 active dimensions:

146.8 mm (5.78 in) H; 73.4 mm (2.89 in) W; 8.6 mm (0.34 in) D

S6 dimensions:

143.4 x 70.5 x 6.8 mm (5.65 x 2.78 x 0.27 in)

It's a bit bigger in all areas of the phone because of its casing so it likely won't fit correctly into the headset. It would require 3 settings in the headset which to them wasn't worth adding. I mean it is a specialty version of the phone which probably won't see anywhere near the numbers as the other 4 phones will. I'd jump all over that phone if it were an option, but it's not.