As per the provided link, there's a number of factors going on.
First of all, a kiai is as much about a forceful exhalation and a focus of purpose as it is actually yelling. Many of the fighters do just that grunting or exhaling on a heavy strike.
One of the other purposes of a kiai is to startle the opponent. In the Octagon, most fighters are prepared for ...
Wrestling and judo were not unknown, but particularly outside of Olympic season generally weren't featured much. Pancrase, shootwrestling etc. were largely a curiosity among pro wrestling enthusiasts.
BJJ was featured in movies like Lethal Weapon but exploded after UFC1.
If there's one thing I've learned from watching lots of UFC/MMA - it's that it is one of the most exciting sports because anyone can beat anyone. Why? knocking someone out is just one good punch away.
If this guy had a natural talent for clocking people on exactly right spot: on their jaw line with a great straight striking technique or accurate haymaker, or ...