Why are dark roast beans usually used for espresso?
Bitter taste is not from the degree of roast. It is from the oils produced when you pass second crack.

There is a way to enjoy a dark roast espresso without the bitter taste: On your machine allow &frac12; a second of liquid to come out before placing the cup. Done, simple. 

The oils are the first thing that comes out of the machine and for only about half a second.