They couldn't call it windows 9 because they realized that when some scripts check to see what OS you are running when accessing a website & check for 9* as in 98 and 95 so if they called it windows 9 it would get mixed up for those instead, but they only realized at the last minute when someone pointed it out to them
As far as I know it will be released as Windows 10 and is supposed to be on Jan 21st.