I have no idea why the launched the xbox 360 with such an obvious design flaw.
Likely the same reasons any retailer puts products out too early: to beat out the competition. The 360/PS3 generation was such a massive improvement graphically that people were dying to get their hands on one, and I expect Microsoft knew the first to be released would gain a massive market share. I don't know if they were aware of the PS3's hardware destined to edge them out (graphics, blu-ray) but they knew how important it was to get that first console out there. You can argue that they should have spotted the flawed heat dissipation earlier, but I suppose by the time it was realized (IF it was realized) it was too late to delay if their market strategy was to beat out Sony.
But I'm just speaking hypothetically with lots of words =P I'm new to Tom's