Here's another reason why...
The number 13 may have been purposely vilified by the founders of patriarchal religions in the early days of western civilization because it represented femininity. Thirteen had been revered in prehistoric goddess-worshiping cultures, we are told, because it corresponded to the number of lunar (menstrual) cycles in a year (13 x 28 = 364 days). The "Earth Mother of Laussel," for example — a 27,000-year-old carving found near the Lascaux caves in France often cited as an icon of matriarchal spirituality — depicts a female figure holding a cresent-shaped horn bearing 13 notches. As the solar calendar triumphed over the lunar with the rise of male-dominated civilization, it is surmised, so did the number 12 over the number 13, thereafter considered anathema.
