I'd say it's because when someone asks 'why' and you want them to stop asking why or you don't really have an explanation, the natural thing to say is just because.
Also, 'therefore' introduces a consequence, not a reason, while 'because' introduces a reason, and 'just because' is used when you've run out of reasons:
He was injured, and therefore he was unable to play. (The fact that he couldn't play is a consequence of his injury)
He was unable to play because he was injured. (The fact that he was injured is the reason why he couldn't play)
Imagine that after the statements in the previous paragraph, someone would ask: and why did the fact that he was injured result in him being unable to play?
You'd answer: *well, because he needs to recover and so, he needs to rest so that he can heal'.
And your interlocutor then asks: yes, but why?
well, just because! I don't know, I'm not a doctor, stop asking so many questions! You're driving me mad!!!
Hope this helps :)