It's purely resources. We are going to assume you mean: Why can't you edit the Insert Effects assigned to a Drum Kit Part while that Part is in a Song Mixing or Pattern Mixing program? (because you certainly can additionally the Insert Effects in Voice mode)

It's because routing to the Part's INSERT EFFECTS is done on a per Element (oscillator) basis. In a Normal Voice there are 8 Elements possible per Voice, but in a Drum Kit Voice there are 73 Elements. So it is purely the size of the Edit Buffer necessary to contain individual routing of the 73 Drum/Perc Elements to one or the other or neither of the dual Insert Effects.

This is one of the reasons the MOXF is about half the price of the Motif XF series. With the Editor (using the computer resources) you can address the Drum Kits for full Edit without leaving the MIXING program.

That's the WHY? If the MOXF had the bigger edit Buffer like the Motif series, you could edit the routings of all 73 Drum Elements right on board without having to exit the MIXING mode.
The HOW... you already know, use the computer Editor to widen the functionality of the editing functions! This allows the less expensive MOXF series to share the benefits of advanced routing of the top-of-the-line instrument, even though you have to use an external Editor to access it. (We think it's a fair trade-off)... and certainly beats the alternative where you don't get to program Drums individually.
Hope that helps.

Thank you! All clear.