Difference is Sisters were an almost exclusivly direct only, metal only army. They got what they needed. If they'd waited to get a book launch, it'd need a full plastic set of minis as well and would have taken much longer
As for Tau....a tiny WD update wouldn't be the worst thing in the world. Have a single page of FAQ in WD that simply says;
"From now on all Tau battlesuits are BS4, Kroot guns are Assault 2, Dead drones don't force a leadership test on units and all Markerlights fire 2 shots and all count as networked."
Short of an actual book thats half a page in WD that would mean the Tau could actually play and actually survive a game.
If they could make Dark Angels and Templars competive (or less crap) with the FAQ that re-dressed points and stuff why can't they do that with Tau? We didn't even get a FAQ in the last round