Quite simple, because it is Love for Allah(swt) that is the guiding force to learn how to submit to His will. Once a Christian discovers that Jesus(as) submitted to the will of Allah(swt) it is only natural to follow Isa(as) into Islam.

Most Muslims that have Christian roots do not give up love for Isa(as), but they have learned to respect him as they respect all the Prophets(Peace Be Upon All of Them).

I believe it can be safe to say that many Christians who revert do feel learning about Christ(as) lead them to Islam and Islam taught them the true nature of Christ(as)