My guess is that .xinitrc checks the users home directory first, then the X11 directories. If the .xinitrc file is empty it has no information about your graphics card and display settings.
Perhaps try copying .xinitrc from it's usual home on the mac to your home directory so all the proper settings and information will be there. That might work. I don't usually have a copy of that file in my home dir, I'm not sure why you would want it there?