The first reason is that Jim Carrey was not Born Into Greatness. During an interview with James Lipton on Inside the Actors Studio, Carrey stated,
Too often we blame our story for why we are where we are, or more importantly why we are not where we want to be. The second reason is when you look at his career progression. Most of us think of Jim Carrey and our minds immediately go to Ace Ventura Pet Detective or, even earlier, In Living Color. Like many actors, he was not landing A+ roles out of the gate. I would even go so far as to say that he didn't have alot of natural talent.
What he did have was the ability to maximize his output on EVERY roll. Call him a ham, but he kicks ass. Imagine if you kicked ass Fire Marshall bill style on everything you did.
This video speaks volumes and is worth the watch.