There are all sorts of reasons why I don't do evangelism.
1. Delusion. I convince myself that it's not necessary. This could be for any number of reasons such as thinking people don't need Jesus, thinking Jesus is unimportant or irrelevant. Denying hell. Being unimpressed with Jesus. As Adam Beattie says - "delusions are lies, we need the truth". Giving me training in skills wont help me to overcome delusions. I need a heart captured by the gospel - changed by the great doctrines of salvation and revelation.
2. Inability. Sometimes I can't work out how to explain things, how to translate into language that can be understood or how to engage with the issues others have. I need some skills training, but mostly I need a more thoroughly Biblical worldview so that I can see how the gospel engages with all of life. And, I need doctrine of grace so I can go and give answers that may not be brilliant but which would be ok. I need freedom to fail.
3. Sin. Sometimes I'm simply disobeying what I know. All disobedience of God's word is sin. I need the Holy Spirit to change my heart. Skills wont help this - my heart is the issue.
4. Courage. I get scared. I get scared of meeting new people. I get scared of speaking about Jesus. Usually this is because I fear people more than Jesus. I fear ruining friendships or being embarrassed. The early church prayed for boldness. That's a prayer I need.
5. Opportunity. Sometimes I don't do evangelism because I can't move for Christians all around me. My job means my colleagues are Christians. My marriage means I live with a Christian. And my Britishness means I dare not talk to my neighbours. Yet, since when was I, as a Christian, meant to be a victim of circumstance. In the power of the Holy Spirit the gospel sends me outwards towards others, to engage them as fellow image-bearers in need of Jesus.
The basic problem in these is my heart. My heart needs the gospel. And by the gospel and for the sake of the gospel change is possible.
Edit: I'd also say, that sometimes I don't do evangelism because I'm actually doing something that is genuinely more important for me to be doing at that moment to bring glory to God, which might be.... eating, sleeping, working or any number of other things. Fact remains, I'm not convinced I get down to shaping my life around introducing others to Jesus anywhere near as much as I might do.