Typically, a short time after a new version of iOS comes out, Apple stops signing the previous one. If you're not familiar with the signing process, after you install a new version of iOS, it checks with Apple's servers to make sure that it's an authentic copy and hasn't been tampered with or modified in any way. That prevents, for example, people being tricked into installing an older version with known security exploits, or an altered, malicious version.
Since iOS 8.1.1 includes numerous bug fixes and performance enhancements, there should be few if any reasons for anyone to want to downgrade anyway.
In fact, if you haven't upgraded to iOS 8 at all yet, iOS 8.1.1 is likely the version you've been waiting for.
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