There are a few reasons why there is no Windows 9.

• To avoid confusion with Windows 95 and 98. Some older software programs still check for the presence of “Windows 9” in the system registry. If Windows 9 had been released, these programs may have malfunctioned.
• To signify a new era for Windows. Windows 8 had been a controversial release, and Microsoft wanted to make it clear that Windows 10 was a significant departure from it.
• To make Windows more evergreen. Microsoft is now planning to release regular updates to Windows 10, rather than releasing new major versions every few years. This means that Windows 10 is likely to be the last major version of Windows.

In a 2015 interview, Microsoft CEO Satya Nadella explained the decision to skip Windows 9:

“This is the beginning of a new era for Windows. Windows 10 will be the last version of Windows. Windows as a service will be the new way we deliver Windows.”

This suggests that Microsoft is committed to keeping Windows 10 up-to-date with new features and security fixes, rather than releasing new major versions of Windows every few years.