Uzbek,
Welcome to the forums. I have only just read your post and want to thank you for it.
When anyone leaves Islam for Christianity or Christianity for Islam, both sides think of it as a major "win" for their side.
IMHO what is happening is that the person who leaves is seeking after a close relationship with God. Anyone can be religious and follow a set of teachings and behaviours, but only a relationship with God will satisfy the hungering spiritual soul of man.
We spend a lot of time, whether Muslim or Christian "doing" things to try and find that relationship. It's very human. Very few people know themselves unconditionally loved and accepted. We know ourselves accepted when we receive a pat on the back. Oftentimes if we are criticised for any reason, we take it very personally. We ache to be accepted.
In religion, we do all that Islam requires to know we are accepted by God according to Isla. In religion we do all that Christianity requires to be accepted by God according to Christianity. What is deep in the heart of man is a yearning, a deep spiritual hunger, that is not filled by doing things for Him, but by "being" in relationship to Him.
As long as religion is the only basis for that relationship, people will swap between denominations, churches and even religions seeking to find that hunger filled.
Jesus talked about this in John 6: 25-69. He said
32Jesus said to them, "I tell you the truth, it is not Moses who has given you the bread from heaven, but it is my Father who gives you the true bread from heaven. 33For the bread of God is he who comes down from heaven and gives life to the world."
You see, no amount of religious activity, no matter how genuine, no matter how much it's based on good works will ever satisfy that hunger... it is God alone who can satisfy you and that is why some people go on searches through all religions. Until they find God through Jesus they will never be satisfied. Jesus said in those same verses
35Then Jesus declared, "I am the bread of life. He who comes to me will never go hungry, and he who believes in me will never be thirsty.
May all those who desire to never hunger again feed on the 'bread of life" alone.
This comes in peace
Carol