In my opinion, it is simply because we as humans are growth-oriented and if you are constantly growing, you are likely to be content for a short period of time.
But additionally, to be content or to feel content maybe be two different things.
to be content would be to be in a state of being content. Whereas to feel content, that is a feeling, an emotional state. That is determined by your thoughts.
But, ironically, you would have to control your thoughts in order to enter a state of being.
So thoughts create emotions. Emotions create energy/feelings. energy/feelings create your state of being.
Now the question is why do you believe that contentedness is difficult to achieve or feel?
How do you define achieving contentedness? or feeling contentedness?
Why do you believe you want to achieve this state or feel this feeling?
What would achieving or feeling this look and feel like for you?
How do you believe one is supposed to reach this state or emotion?
What stops you from reaching this state or emotion?
Hope the above helps you.