What would you say is the cause of the decline of practical discipleship?
I don't think there is a single cause. Like most things in the human experience, there are multiple forces at work.
In the mainstream denominations that are not particularly evangelistic, I would attribute much of the decline to disbelief in the reality and efficacy of the resurrected Christ.
In non-charismatic circles (both evangelistic and non-evangelistic), I think there was a reaction against the immanent reality of the Person of Christ and the power of the Holy Spirit as a reaction against the excesses of the movements that emphasized speaking in tongues, divine healing for every disease and ache, and the rise of the anti-Christian prosperity movement that infiltrated the church through people like Kenneth Hagin and Oral Roberts. If, in the well-intentioned effort to protect the people in the pews from these excesses, we essentially tell people that God is not going to act directly in their lives and that the only true experiences they will have will be mediated through bible study and preaching, then they will get the idea that mental assent (not intentional transformational disciplines worked out in ones life) is the key to Christian living.
In denominations and movements that emphasize evangelism (which I fully support), there has been a tendency to turn evangelism into a 1940s-50s style door-to-door sales technique that hammers a few basic theological propositions (e.g. "God loves you and has a wonderful plan for your life" and tries to close the sale). While the intent is good and much of the time, the theological propositions may be sound, it has the effect of reducing the gospel to a couple of theological points instead of the broader call of Jesus - a call of transformation and discipleship. Even this theological missive that started this discussion does not seem to acknowledge that there is anything more to the gospel than the eternal destination of individuals, to either heaven or hell. While there is enormous value in being succinct in an evangelistic presentation, there has been an astonishing lack of theological depth in the training of both the people in the pulpit and in the pew regarding the way Jesus, Paul, Peter, and the church as recorded in the Book of Acts evangelized their culture and the other cultures they encountered. Furthermore, evangelism was not isolated from other aspects of the Christian life. The lifestyles of the evangelists themselves authenticated the messages that they delivered to their hearers. BECAUSE their lives were extraordinary, the gospel message that they presented captured the attention of their hearers. Because their lives were being transformed through active practices of discipleship, they were very sensitive to the leading of the Holy Spirit and had practical faith to trust the Spirit's power and leadership as they faced each situation. That kind of faith is gained and grown through discipleship.
In Southern Baptist circles, I think the decline of "Training Union" in the 1960s-70s also contributed to the decline. In the church where I grew up, by the late 1970s, Training Union amounted to a Sunday night session of an ill-informed teacher telling us how bad Roman Catholics were and how they were going straight to hell. Having some Roman Catholics in my extended family (and my mother being a former Roman Catholic), I knew that a lot of what I was being told was a misunderstanding at best, or outright lies at worst. While I have no love or appreciation for the Roman Catholic church, it undermined the credibility of the entire Training Union program when it descended to the point of simply bashing Roman Catholics since the teachers didn't really know much about the Bible.
I'm sure there are most elements and movements involved in the decline, but these are just the first few that come to mind.