Windows RT is Windows for ARM devices other than smartphones. Rather than using the Windows Phone 8 operating system for tablets, Microsoft develop a new one. This gives Microsoft three operating systems. Windows Phone 8 which is designed for use with ARM processors and for use on smartphones. Windows 8 which is designed for use with x86 (Intel and AMD) processors and for use on desktops and hybrid devices. Windows RT which is designed for use with ARM processors and for use on tablets and hybrid devices. All three versions of Windows use a common interface, and porting applications from one to another is supposedly not too difficult.
The approach taken is different from Apple, which has a mobile operating for phones and tablets in iOS which is compatible with ARM processors, and a desktop operating system which is compatible with x86 processors. The approach is also different from Google, which has Android for phones and tablets, and Chrome OS for desktops. Android and Chrome are compatible with both ARM and x86 processors.
The advantage of ARM was that it was the lower cost solution, and had good battery life. ARM processor also did not have the power to run a full desktop operating system. The death of Windows RT is due mainly to two factors:
1. Intel has closed the gap with ARM. Intel Atom processors can run the full Windows 8 operating system on a tablet, and give battery life more or less equivalent to other tablets. Basically, why run RT and when can run full Windows 8. With Intel's new Haswell, the need for Windows RT will be even less.
2. Microsoft kept too tight a control of ARM. Microsoft imposed tight controls on the development and production of Windows RT devices. All Windows RT devices were designed in cooperation with Microsoft. All Windows RT devices were built to strict design and hardware specifications provided by Microsoft. Three participating ARM chip makers were only allowed to partner with up to two PC manufacturers to develop the first wave of Windows RT devices. Qualcomm partnered with Samsung and HP, Nvidia with Asus and Lenovo, and Texas Instruments with Toshiba. Additionally, Microsoft partnered with Nvidia to produce its own Windows RT tablet, known as Surface.
Acer and Sony did not participate, whether by decision of Microsoft or the OEM's, I do not know. HTC was not allowed to build a Windows RT tablet. Lower cost ARM component manufacturers were also left out.
The end result was only five Windows RT devices being released, with Toshiba canceling their offering before launch. Effectively, there are only four left with Samsung canceling their ATIV Tab after release.
Acer built a Windows 8 tablet, and so did Asus, Dell, HP, Lenovo and Samsung. These devices matched the prices and the features of the five magical Windows RT devices. Windows RT on ARM might actually have been able to provide lower cost Windows devices.
Windows RT already had the daunting task of trying to take out the Apple iPad. The end result was a situation where instead of Windows RT competing with the iPad and Android tablets, Windows RT's first competitor was Windows 8. Apple iPad's and Android tablets, were at a different and lower price point all-together.
Basically, when you read about that booming tablet environment, most of it is being done be devices that cost less than Windows RT and Windows 8 tablets. For Windows 8 tablets, this is to be expected. They are premium products which will be the choice of those who need Windows XP/Vista/7 compatible software on a tablet.
Windows RT, needed mass acceptance to succeed. Without a large user base it would not be able to attract app developers. Without a large app ecosystem, it really has little hope of competing with the iPad.
In sum, Intel rendered Windows RT irrelevant with advancements in its processors. Microsoft rendered Windows RT irrelevant by creating a environment for its development which resulted in it coming into the market as a premium product.
Microsoft has the most operating systems, and many OEM partners, collectively they have not come out with a single product that can match the price of an Apple iPad 2. Never mind the iPad mini, Android and Kindle tablets sold cheaper than that. Instead Microsoft, leveraging MS Office, tried to go head-on with the Apple iPad Retina, and its 300,000 app ecosystem.
The scariest message Windows RT send Microsoft is that, "We can live without MS Office."