Whether espresso or drip coffee has more complex flavors really depends on who you ask. However, if your coffee has more complex flavors, it's because it has more thorough extraction. The time it takes for a standard cup of coffee to brew allows for a wider range of compounds, all with slightly different flavors, to be extracted from the coffee grounds. However, this does mean that coffee captures more of the sourness associated with the lower amounts of extraction and the bitterness associated with the higher ends of extraction. A good brewing system does a great job of balancing this.