They seem so easy to land. And they're so quick to land and throw. And they're so powerful. They seem so obvious. Why has no one ever thrown these with bad intentions before?
Other fighters have used shoulder strikes before. It is just rare for a fighter to lunge in from so far out multiple times without their opponent making any adjustments.
A shoulder strike is primarily used to either create distance or to stun an opponent or to do both simultaneously. A shoulder strike can be used to set up powerful strikes or to initiate or be part of a combination. It is something that can disrupt an opponent. It could be used to lift the opponents head and to create space to land hooks, shovel hooks, uppercuts and over hand punches. It could be used to create some space to attempt to land hooks/shovel hooks to the liver or solar plexus.
Another in fighting technique that is used is a forearm strike that can also be used to create space or to position an opponents head.
There are plenty of in fighting techniques that are under utilized.
There are also plenty of "dirty boxing" techniques that are perfectly legal in mma.
The reason why you don't see more of shoulder strikes like McGregor on Cerrone is that most opponents make adjustments and that particular technique can work to a fighters detriment. A fighter can anticipate that strike and use that fighters momentum against them. Cerrone had an underhook with his left hand. He could have used that to pivot when McGregor leaped in. He could have used that to suplex McGregor or simply to knock him off balance to set up a combination of strikes himself. He could have possibly even attempted to time an elbow strike to catch McGregor as he leaped in.
But I guess the specific answer to your question is that a shoulder strike has a fairly low probability (relatively speaking) of creating a knockout. On a single strike basis it seems far more probable that a punch, elbow, knee or kick could create a knockout over a shoulder strike. Therefore it is arguable that rather than attempting to throw multiple shoulder strikes in succession it seems like it is a better strategy with a far higher probability of payoff to use shoulder strikes to create opportunities for some combination of punches, elbows, knees and kicks.
