Backspace/Delete ⌫ no longer works as a back button in Safari 6.
How to make the Backspace key to take me to the previous page in Safari 6?
I actually asked a similar question (but more broad so this isn't a duplicate). I got this as an answer (it works, I tested it):
Write the below line in terminal:
defaults write com.apple.Safari com.apple.Safari.ContentPageGroupIdentifier.WebKit2BackspaceKeyNavigationEnabled -bool YES
Then restart Safari.
Hope this helps!
Apple removed the backspace (probably to prevent users that wanted to edit a form field to accidentally go to the previous page)
You've got a number of options: