Why won't my iPhone 5S downgrade to IOS 7.1.1?

I updated my iPhone to IOS8 beta 2 a few days ago, and found lots of bugs which are starting to bother me, so I tried to downgrade to IOS 7.1.1 using the IOS 7.1.1 ipsw file for my iPhone GSM (A1457) phone, but everytime I try to restore it using iTunes, I get the message "the iPhone could not be updated because the firmware file is not compatible", so after that i decided to download another ipsw file from another source and it said the same thing, and I just don't know what the problem is.

You can't downgrade to any ios x