Because he will be replacing the Paladinosaur as the new Loa of Kings, and there are so many possible implications to that.
Like, will Vol’jin become the new power source of Zandalari Paladins?
And will the general population of Zandalari trolls accept to make a “lesser troll” their main god?
I think there’s a lot of potential to be explored in this storyline 
Shirvallah is another decent contender but is starting to feel like a jack of trades with classes. 
I think it makes sense.
Vol’jin was powerful beyond just his ability as a troll, able to talk to Talanji (and already greatly respected by the Zandalari at this stage) during the 4th war and essentially ‘merging’ with Rezan will give him even more power after he is reincarnated.
Vol’jin will exist but he will be a speaker of Kings and essentially wield all the same powers and more than Dino boy.
Trolls that worship Rezan will still worship Rezan, but ack Vol’jin as his speaker. Think of him as Magni is for Azeroth.