Because of the reasons already mentioned twice above 
(see the solution in post #18)
You have given the container a height of 100% and therefore it can never grow any more than 100% which in the case of your absolute element is one viewport height. As Ralph mentioned height:100% is not something that you will ever need to apply apart from on html and body when you want a min-height:100% wrapper.
I would remove the absolute positioning also as you don’t need it and will preclude you from adding any content underneath (such as a footer if required).