I believe the problem is with the roots. It is potted in a very unusual and overly large container that I suspect may not have drain holes. That is a recipe for root suffocation. When the roots are in trouble, then they are no longer able to supply the plant with the water and nutrients that it needs and the plant then dies. It looks like yours is too far gone to be saved.
Out of curiosity, when and why was it put into that container?