Why the name derives for Windows 7 as Windows 7

First Windows version was Windows 1.0. Second was Windows 2.0 and third was Windows 3.0. When Windows NT was released, it was code versioned as Windows 3.1. Windows 95, 98, 98 SE and ME (Millennium Edition) were code versioned as Windows 4.0 as all were using Non-NT kernel or 9x kernel.
Windows 2000 was code versioned Windows

5.0 and Windows XP as Windows 5.1. Next version was Windows Vista which was code versioned as Windows 6.0. Since Windows 7 is the next Windows version, Microsoft decided to call it Windows 7 for easy and better understanding.

Short Note:

1.0: Windows 1.0
2.0: Windows 2.0
3.0: Windows 3.0
3.1: Windows NT
4.0: Windows 95, 98, 98SE, ME
5.0: Windows 2000
5.1: Windows XP
6.0: Windows Vista
7.0: Windows 7