The Fitbit Charge is not meant to be the successor of the Fitbit Flex. When it comes to physical appearance, Fitbit Flex is smaller and doesn’t have a display whereas Fitbit Charge is two times bigger and offer a OLED display. Both serve different target audiences and that is the reason why Fitbit Flex is still on sale.
*UPDATE 10th December 2014* Looks like Christmas came early for those waiting for the Fitbit Charge. It will start selling in Singapore from 19th December 2014. Fitbit Charge HR and Fitbit Surge are still on track for an early 2015 launch in Singapore.