JWs associate luck with demonic worship, hence they do not use that expression. Demons really do not exist--for example Jews do not believe snake that spoke in the garden of Eden is Satan. In the temptation account involving Jesus, Satan is shown as somebody who believes the shape of earth is flat because he takes Jesus to the top of the mountain to show "all the kingdoms of the earth." Hence Bible too shows that Satan is symbolic of sin, not a real person. If Satan is real, he would have caused an accident where ever nuclear weapons are kept and would have already caused the destruction of the humanity. Hence modern history too shows that Satan is not real, hence his supposed supporters called demons too are not real.
Hence you can use the expression luck because luck exists. For a high-profile job opportunity, suppose two equally good people are finally short-listed, yet one only will be selected--that one is the lucky one.
Another example, my friend was to board a plane, but was prevented from doing so because of a spelling mistake found in his passport and other documents. But five minutes after take off, plane had a mid-air collision with another plane leaving no survivors. My friend's apparent bad luck turned out to be a real luck.