This idiom here is used each and every day. Please use it any time.
"How come?"
It means, Why?
Example 1
A. I'm so excited!
B. How come?
A. I'm going to Hawaii tomorrow!
Example 2
A. I can not go to your party tomorrow night.
B. How come?
A. I have to work late, again.
Example 3
A. How come you got an invitation and I didn't?
B. I don't know.
https://idioms.thefreedictionary.com/how+come