One big reason why Nokia is closing up the Symbian shop is that it takes 22 months for a Symbian powered phone to go from napkin to shipped product. Windows Phone models take less than a year to go from drawing board to a buyer's hand. Last year, the manufacturer released seven new Windows Phone models in just seven months.
With Nokia beginning to position its high end Windows Phone models as the ones to choose for great low-light photography, it looks like control of this small niche of the market is what Nokia hopes is just the beginning of Windows Phone's increasing market share in the months ahead. As for Symbian, it served Nokia well and was once the most popular smartphone OS until Android took the crown in the last quarter of 2010.
source: FinancialTimes (subscription req'd) via WMPoweruser