In der Schule haben wir gelernt: Vor because kein Komma.

I have no idea what the rule is, but the comma splits the sentence up and makes it easy to read.

http://www.bartleby.com/64/C001/015.html

because after negated verbs.  When because follows a negated verb phrase, it must be preceded by a comma when the because clause explains why the event did not occur. They didn't want her on the committee, because she was so outspoken means roughly "Her outspokenness was their reason for not wanting her on the committee." When there is no comma, the because clause is included in what is being negated. Thus They didn't want her on the committee because she was so outspoken implies that they may in fact have wanted her on the committee but for some reason other than her outspokenness.