I could list so many protos that never saw the light of day it would be ridiculous.
Kuill has the distinction of being an alien AND having a short body type, both of which HT has not done very often. He also doesn't have the following in the broader world that he does on this board. Someone posted that Kuill wasn't getting much love on Facebook back when he was revealed. It is very possible that HT teased him to see the response... and its very possible the response just wasn't strong enough based on how they interpret response. And I doubt the Blurrg sold very well at all.
My opinion is that Kuill will go the way of so many others that have been teased and toured for years at shows and still never made the cut to get made. That's just my opinion. It could very well be the next Krennic. I just doubt it. Krennic got made because of easy reuse. Kuill... alien, complicated outfit on a little body. Too much work, too little pay-off.
I would be very happy to be wrong. Believe me.