Why Is Writing a Prospecting Letter More Challenging Than a Solicited

Why is writing a prospecting letter more challenging than a solicited letter when sending a résumé?

A) You have less research on the company
B) You don't know what skills the company is most interested in
C) You don't have a clear target as you have with a solicited letter
D) It is hard to find the name of a person to send the letter to
E) You have no common ground with the reader