Windows 8 is developed to compete with mobile operating systems such as Android and iOS. Microsoft has been ruling the OS platforms. Windows 8 has drastic changes in platform and user interface of the operating systems. Microsoft is not new to mobile devices. It had a smartphone before the Apple iPhone revolution came along, and it was pushing tablet PCs before the Apple iPad made it cool. But, as long as Microsoft’s history with mobile devices is, so is its stubborn desire to make everything about its Windows OS.

There is a radical change in Windows 8 when compared to Microsoft operating systems from the launch of Windows 95. Microsoft currently has a mess on its hands with Windows 8. The tile-based, touch-optimized OS was supposed to propel Microsoft into a competitive position against Apple and Android vendors in the tablet market.

Considering the software or OS to be user friendly is one of the major factors for its success and also to attract more customers.
I personally don't feel it is possible to develop single OS which can be efficient on tablets and PC's. Windows 8 is developed to serve as OS in desktop as well as mobile which is why it is touch screen friendly but not effective on desktop.

Windows 8 is a personal computer operating system developed by Microsoft. Development of Windows 8 started before the release of its predecessor, Windows 7, in 2009. It was announced at CES 2011, and followed by the release of three pre-release versions from September 2011 to May 2012. The operating system was released to manufacturing on August 1, 2012, and was released for general availability on October 26, 2012.[3]
Windows 8 introduced major changes to the operating system's platform and user interface to improve its user experience on devices such as tablets, where Windows was now competing with mobile operating systems such as Android and iOS.[4] In particular, these changes included a touch-optimized Windows shell based on Microsoft's "Metro" design language, the Start screen (which displays programs and dynamically updated content on a grid of tiles), a new platform for developing apps with an emphasis on touchscreen input, integration with online services (including the ability to sync apps and settings between devices), and Windows Store, an online store for downloading and purchasing new software. Windows 8 added support for new and emerging technologies such as USB 3.0, Advanced Format hard drives, near field communications, and cloud computing. Additional security features were introduced, such as built-in antivirus software, integration with Microsoft SmartScreen phishing filtering service and support for UEFI Secure Boot on supported devices with