Each device has a unique ID that when you upgrade to the latest iOS version, gets digitally signed and logged by Apple. This has the benefits of ensuring that the version of iOS you have installed is legitimate, secure and free of malware. When a new iOS version gets released, there is a grace period of a few days where Apple will allow people to downgrade back to their previous version, in case of errors or problems. Once this grace period expires, Apple removes the digital signature from the older version of iOS and this means it can not be downloaded or installed on your device. Whenever your Apple device 'phones home', Apple check to make sure the digital signature is still correct.
You are stuck on the version you have and there is no going back. Sorry to be the bearer of bad news.
03-29-2015 02:21 PM