Tried updating to ios 6
On every device shipped by Apple a Plist file is present, this file contains SHSH blobs for different parts of the firmware.
These blobs are essentially gatekeepers and control whether or not a firmware is restorable.
In order to enforce this limitation SHSH blobs were introduced, which is a hash signature system.
Although this is getting technical, an SHSH is created by an SHSH formula with 3 or 4 TSS keys (say with me).
Downgrading your i Phone, i Pod Touch or i Pad to a lower firmware version is indeed possible, but unfortunately has become harder and harder over the years.
Just like how Jailbreaking is becoming more and more difficult on newer Apple devices, so is downgrading.