Microsoft skipped the number 9 because they felt like it.
Windows 10 is this dope new thing where Microsoft basically takes everything from Windows 8/8.1 and crams it into the Windows 7 desktop.

Notes:
• The old Start menu is back, now sporting the Live Tiles from Windows 8, Windows Phone and Xbox.
• Windows Store apps can now be run without needing to be fullscreen.
• Microsoft is pushing this "One Windows" thing which means Windows 10 for phones, tablets, laptops, desktops, Xbox One and other devices (aka buy an app once and run it on all of these).
• And something I've noticed, in Microsoft's presentation you can see the Instagram for Windows Phone icon which might mean that Windows 10 is backwards compatible with not just Windows 8 apps but also Windows Phone apps.

(Source: Blogs.Windows.com)