German phrase
Ich weiß nicht, warum ~.
Meaning
I don't know why ___
When to use it
You do not know the reason. It is neutral and common in daily conversation.
Example dialogue
Ich weiß nicht, warum er so spät kommt.
I don't know why he is so late.
Vielleicht hat er den Bus verpasst.
Maybe he missed the bus.