Because viruses will survive so easily in the lush organic environments on Malachor. And G0-T0 has all those well-established biological weapons facilities we've all forgotten about.
See what I mean about G0-T0's options getting increasingly hare-brained and desperate?
Let me get this straight you're saying that: if G0-T0 manages to send a bunch of probe droids to Malachor V, carrying canisters containing an airborne pathogen, and they manage to get it into Traya's academy
(especially if the virus has about a 24 hour time frame before people start manifesting the symptoms; is a hare-brained and desperate idea?
Also concerning the droids, yes shrapnel is an issue, but it isn't as big of an issue for a droid as it would be for someone in a space suit.