They don't mean the same thing. http://www.oxforddictionaries.com/definition/english/hillside
It could just be a colloquial quirk here to use them interchangeably. So assuming that ‘hill’ and ‘hillside’ are different (and hills are trapezia now :P), all of the dictionaries below, plus Google Translate and my own Hippocrene Practical Norwegian Dictionary, translate ‘bakke’ as ‘hill’, none give ‘hillside’ as an option. Then, back to the original question, why is ‘the hill’ not accepted as a correct translation for ‘bakken’?
"The hill..." was added as a possible translation last week so I'm not sure why it wasn't accepted in your case. But I just wanted to point out that the use of the word 'bakke' will be somewhat different than the use of 'hill' in English. 'hill' usually have a top, but this is not the case with 'bakke', which just refer to the slope.
This sentence would probably not refer to a hillside, but to the ground. But that will depend on context.
They buried me in the cold, hard earth. There were worms in that earth. I hate worms. Worms drive me crazy. Crazy? I was crazy once. They put me in a little, yellow, rubber room. I died in that room and they buried me in the cold, hard earth. There were worms in that earth. I hate worms. Worms drive me crazy. Crazy? I was crazy once...