Gold has unique physical properties. It "backs" itself.
Backing btc requires a central organization to implement, which defeats the purpose of btc.
Bitcoins can be backed and still be decentralised just like it is now. Nothing would have to change with the transactions. The only thing centralized would be the backing and that could also be decentralized to some degree. Anyway, if the backing worked your better off with it than without it, and if the backing failed it would be the same as if you didn't have it.