I was just wondering: why is it considered a bad luck to say " Macbeth" in Shakespeare's Macbeth?

It's not actually bad luck to say "Macbeth" during a
production of the work.  Superstition has it that it is bad luck to refer to the play by
name; hence those who refer to "the Scottish play" or other
phrases.

The reasoning behind the superstition is that
Shakespeare used actual witches spells in the play, with the result being that the
witches of his day cursed the play.  As a result of the curse, productions of that play
may be doomed if specifically named inside the theater, or performers involved in the
production may be harmed.

In order to prevent the curse
from coming to pass, those who are worried about the superstition might go through
rituals involving leaving the building, spinning three times in place, spitting over
one's shoulder, and uttering an obscenity.