Because to peg a sidechain to the mainchain, someone, centralised or in a decentralised manner, must verify that transaction met some conditions (e.g.,to lock the coins on mainchain and is certain levels deep). If the transaction is malleable, it is difficult to programatically achieve this since txid could change. I think depending on the design of the sidechain, this can be taken care of, BUT although the solution might be programmable, it would most probably be centralised.