I'm just speculating, but maybe the car type buckle can't survive the higher g forces in an accident. Aircraft buckles are designed to keep you in your seat to a fairly high crash (g) force. Also aircraft latch type buckles seem simpler mechanically than car button type buckles - less expensive. And less likely to jam, sure - can you imagine the dilemmas on board if passengers couldn't get their button tyhpe buckles to unlatch and disembark?