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Two reasons for this are: 1) unbelief; and 2) an incomplete view of prayer.
Unbelief is one reason for a prayerless life. If prayer means talking with God about what we are doing together, then how could anyone pass up daily opportunities to meet, one-on-one, with the Maker of Heaven and Earth? I can assure you that, if right now the President of the United States (or any country's President) called and said they wanted to meet with me today, I would stop typing this post, and say "Excuse me, I have a meeting with our President." I would drop all things to do this! A chance to meet with the most powerful leader in the world! You would not be able to keep me from such a meeting. And, I would go with awe and trembling.
Multiply this unlikely earthly scenario times a gazillion and we have the matter of prayer as meeting with the all-powerful, all-knowing, necessarily existent, Creator of all things. Almighty God invites you to pray today, which means to enter into conversation with Him concerning many things, to include the Kingdom Mission. If you can't find time for this I suggest it may it be because you don't believe.
Another reason "Christians" don't actually pray is because they have been taught an incomplete, one-sided theory of prayer. This is the idea of prayer as essentially "asking" or "petition." This is found in, e.g., the theology of Karl Barth, who so much emphasized the "Wholly Otherness" of God that God got viewed as phenomenally distant. So we talk to Him more than converse with Him. We come to God mostly with requests. We approach this distant God when we're in trouble.
I know there's more to Barth than this. But this was his emphasis. See how this is expressed in, e.g., the Barthianism and Calvinism of Donald Bloesch, especially his book The Struggle of Prayer. I had Don (who was a great theologian, a very good person, a passionate lover of Jesus, and graciously agreed to speak to my seminary class)) come to speak once in a class I was teaching on prayer at Northern Baptist Theological Seminary. The emphasis was, for me, too much on speaking to God but not enough on hearing from God.
If a Jesus-follower thought "God won't speak to me" this would hugely discourage them from praying.
I like how Anglican theologian Kenneth Leech writes about this. Leech says: "Many people see prayer as asking God for things, pleading with a remote Being about the needs and crises of earth. sometimes these pleas produce a response; often, they do not. So prayer is seen in essentially functional terms - is it effective or not? Does it produce results?... But in order to pray well we need to disengage ourselves from this way of thinking." (Leech, True Prayer, 7)
This is the myth of "effective prayer," with "effectiveness as some kind of measuring stick. To focus on the "effectiveness" of prayer is to miss the relationship with God. It is to view God as some object from which to "get results."
How can we help people who "can't find time to pray" because they don't believe? My view is that only God can change their hearts about this. We should not try to force this on someone. We can create opportunities and contexts for others to encounter God. When I send people out to pray as an assignment in my seminary courses, some become believers (in a God who has much to say to them) and get a prayer life that lasts for a lifetime.
We can also introduce the idea that true prayer is about a conversational relationship rather than simply a 9-1-1 call.