The dicots are thought to be older group of plants from which the monocots have evolved. Thus, monocots are thought to be evolved later than dicots.
The simpler anatomy of the monocots is thought to be more efficient in utilising the solar energy and grow rapidly.
The monocots have the higher ability to withstand damage due to grazing, burning and disease than most of the dicots.
The monocots have only one cotyledon while as dicots have two cotyledons. Thus, monocot embryo gets everything from a single source i.e one cotyledon. The dicot embryo uses more resources to create second cotyledon, thus making it less efficient than the monocot.