It's because when they were together they showed great glimpses but were physically overpowered. Way too overpowered at times. It's not difficult to see why they are not together.
Batherson is not on the top line to overstack it.. but because Tkachuk provides excellent physical complementary play to give Batherson more space.
I can very much see Stutzle-Batherson as our duo of the future if/when they both become physically independent, or if we get an external LW from draft or free agency that will provide serious defensive and physical relief to that line.