Check out this Google Trends chart. It goes from the initial announcement of Windows 8 in September 2011 to the current month, January 2015 and shows the history of searches for “Windows 9” and “Windows 10″.
Nobody was searching for Windows 10 until it was formally announced in September 2014.
By skipping Windows 9 entirely, it allowed Microsoft to start from scratch and to purge all Google-able bad press from their new OS.
Very clever, Microsoft marketing team. Very clever.