Hexagram in a Mosque

Al-Ghouri mosque Cairo ,1356
CC Picture by Dr. Pat (c) from Flickr

Jewish Stained Glass

This Jewish Stained Glass picture is courtesy of "jgoldpac' from Flickr. 

The Star of David is first of all a geometric shape and this background of rhombs makes this point very clear.

In the Shadow of the Holocaust

Picture is courtesy of "Coblat" from Flickr who shot it inside a monument to the Holocaust in Poland and wrote under it:

The star is a Star of David, or in Hebrew, Magen David, which means "Protector of David."
I found this sadly ironic and symbolic.

In other words the question is why didn't the Shield of David, which was created for protecting the Jewish people fulfill its destination when it was extremely needed?