Because it was unbelievable to me at that time that that could be so.Why you stopped believing in God because of that?
Also, do you think you would react differently if you knew at that time that it was the sole interpretation of your instructor or a certain holy book?
I doubt I would have reacted differently knowing that it was simply how my instructor intrepreted it, and that there were other interpretations available. To a 6 year old, the Sunday School instructor is an authority on the matter.