Because he was born in Germany and only was in England because of his mother's uni? The fact that he ever played for England just shows the incompetence of the german FA.
It kinda makes me sad to watch these Videos explaining why a Player chose to play for Germany with 4 worldcups instead of England, he never was born in, with 1 WC
It's not like he rejected England. He thought long and hard, but what made sense was to play for the country he felt he belonged closer to. Which was Germany given his mom is German, he was born in Germany and lived there for 7 years. And by 16 he once again was living in Germany. That's why.