Caramelization is dependent upon the temperature of the sugars reaching their conversion temperature. According to Wikipedia, maltose requires over 350F to caramelize. During a typical boil, obviously the temperature of the system is pretty much at the boiling point temperature. However with a more vigorous boil, there could be teeny points within each of those steam bubbles that might reach a temperature well beyond the boiling point. But that is just for a moment and its effect has to be minor.
However as wort is concentrated, the specific heat value goes down and the boiling point temperature goes up. That 350F temperature criterion will come within reach with concentration. It quickly becomes possible to caramelize an entire mass of a sugar solution instead of a few molecules per steam bubble when you concentrate the wort.