The main reason is that vBulletin doesn't have that feature by default. I am hoping to get vBulletin 4 moving soon, which would break any add-ons that we might install right now.
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.