The reason is because higher rank recipes stay orange longer, so you get guaranteed skill points longer. The rank 2 of this recipe turns yellow at 135, while the rank 3 is orange up to 145. The rank 3 also cost fewer materials, so you will save a lot of materials by getting the rank 3. You could actually level to 150 with the rank 2 recipe, but it will be green for the last 8 points, so you will have to spend a lot of extra materials, and there is no reason to rush 150, so I recommend to wait until you get the rank 3.
Vol’Dun is the hardest and most tedious of the Horde Zones in Battle for Azeroth. It’s also probably the one that makes the most sense. Each sections of quests leads directly to the one you should do next. For all of these reasons it’s going to be best to get this one done early. Thanks to any legendary items you might have collected in Legion, you’ll be strongest in the leveling process before 115 and weaker after — because that’s when the legendary effects stop working.
For reference, my previous best time abusing Method-0 in 7.3.5 was 32.4 hours(as show in the old thread). A difference of just over 8 hours; which isn't all that bad, all things considered. And I believe I made several errors along the way, which could be cleaned up in future runs. I believe that that gap could be significantly closed by using a monk's bonus, but I hate playing monk...so...yeah. YMMV