Because 7 10 11
The posters.
It was low hanging fruit.
Hey Microsoft, why is there no Windows 9? You've given us 7 and 8, and now you're skipping straight to 10?" "Well, to be completely honest, 7 8 9."
Because 7 was a registered 6 offender
7-Eleven on the dot.
Because 7 ate(8) 9!