During my stay in the US I used to talk with children a lot, mainly because of my job responsibilities. The thing is that the majority of those kids under 10 years of age used to say 'How come...?' instead of traditional 'Why...?'. At the same time I do understand that it is a bit of slang, but still ... Wouldn't it be easier for kids to say 'Why...?'? Meaning is almost the same in the matter.
So, how come?)