Why can't I go back with backspace in Safari 8?

Not sure if you are asking what they are in Safari.

CMD + [ Go back (or cmd+alt+5)

CMD + ] Go forward (or cmd+alt+6)

But you can make it work using Terminal with following (Thanks to @Tetsujin research)

Terminal command to re-enable Backspace/Delete as "Back-Button":

defaults write com.apple.Safari com.apple.Safari.ContentPageGroupIdentifier.WebKit2BackspaceKeyNavigationEnabled -bool YES

Quit Safari, then copy & paste the command in a Terminal window (Applications → Utilities) and hit enter. Quit Terminal and restart Safari.

Terminal command to undo the set preference:

defaults write com.apple.Safari com.apple.Safari.ContentPageGroupIdentifier.WebKit2BackspaceKeyNavigationEnabled -bool NO

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