It isn't really unlucky. Lots of attempts have been made to prove whether there is any actual bad luck connected with the number 13 and no link has been found. Yet the fear persists, especially in western countries, and even more so of Friday the 13th. That there is even a name for it - triskadekaphobia or fear of the number 13.

A popular answer to the question of why we have this fear is that there were 13 people at the Last Supper in the Bible. But the fear is older than that. It has been suggested that it goes back to more ancient religions, and one possible explanation can be found here: in many of the oldest cultures, eg Egypt and China, 13 was once thought to be lucky.

It often happens that things that are seen as good in one culture become negative in another, especially when a new society or religion is trying to replace an old one (like the serpent, which was a figure of wisdom in many ancient religions but a symbol of evil in the later Judao-Christian ones.)