Why has the credibility of preaching declined?

J.I. Packer asks why preaching is not seen to be the force that it used to be, and suggests the following answers:-
1. The prevalence of non-preaching in Christian pulpits has eroded awareness of what true preaching is. Good models are rare. Sermons are often built on wrong principles: some have failed to open up Scripture; some have expounded the Bible without applying it; some have addressed only the felt needs of the listeners without confronting them with the Word of god; some have been mere statements of the preacher’s own opinion, rather than messages from God carrying divine authority.
2. Topical rather than textual preaching has become common (in the US more so than in the UK). Themes, rather than texts, are announced partly to make the sermons appear more interesting and relevant, and partly because the preacher may not have confidence that the Word can speak for itself. In the topical sermon, the text is reduced to a peg on which the preacher hangs his own opinion. In such preaching, the authority of the preacher replaces the authority of the Word of God.
3. Low expectations have become self-fulfilling. Few modern hearers expect sermons to matter much. It takes an expectant, praying congregation, as well as a preacher who knows what he is about, to make an authentic preaching ‘occasion’. The key evaluation question has become, ‘How did the preacher get on?’ rather than, ‘How did I, the hearer, get on under the word?’ Hearers are now observers, assessing the performance of the preacher, rather than participants waiting for the Word of God. Those who seek little find little.
4. Persuasive speech has become suspect in our culture. Largely due to the media, relaxed and chatty intimacy has become the standard for communicative sincerity. In former centuries, preacher could use passionate words, tones, and gestures to match the gravity of their message. Such methods are now generally disapproved, with the effect that preachers are reduced to dealing with the great issues of eternity as if they were announcing the sports results. Preachers today would do well to cultivate an honesty with words that earns us the right to fly in the face of our laid-back culture and to dwell passionately, urgently, dramatically, and at appropriate length, on the desperately important agenda of the relaitonship between God and man.
5. Spiritual issues are felt to be irrelevant to many church members: issues such as radical repentance, self-despairing faith, costly cross-bearing as central to discipleship, spending and being spent in order to do good, putting holiness before happiness, and keeping the world out of one’s head and heart. The problem is that for many, church attendance has little to do with the quest for God. They are in church for other reasons. ‘So when preachers point the way to a richer relationship with God, this type of hearer feels a sense of irrelevance, and his or her heart is inclined to say: here is a religious professional talking about the things he is paid to talk about; I am not a religious professional, so none of that is really my business; however, I will sit through it patiently, as good manners require.’
Summarising J.I. Packer, ‘From the Scriptures to the Sermon’, in Collected Shorter Writings, Vol 3, 280-283.