In short, it is more difficult to preach than it is to lecture. Good application is rare because preachers assume their hearers can and will make the application. While most hearers are concerned about their own needs, they lack either the ability or the inclination to relate the Word of God to those needs. Like the Ethiopian eunuch, they need someone to guide them, taking the principles that Paul espouses and applying them to modern America in the same fashion that Paul used them to impact Corinth. Without application, the hearers may nod in agreement, but more likely they will nod off in apathy.
Good application is rare because it requires creativity and courage.
Creativity is required to envision the battles fought daily in the lives of God's people. Courage is required to apply God's Word to those skirmishes on a personal level. Because application concentrates on the transformation God requires of his people, hearers, unwilling to directly attack the message, often attack the messenger. Therefore, the messenger is tempted either to mince words or avoid the application. Good application is rare because preachers fear being too simple.