why OP_CHECKMULTISIG signatures must be placed in the signature script using the same order
Performance. 

If you have an m of n multisig scheme, ordering the keys and the signatures  ensures that a minimum of m and a maximum of n signatures will be verified.

If we didn't use this approach the number of signatures to be verified could be as high as  n! / (n-m)! ( this is the worst case scenario).

Keep in mind that, in terms of resources, it is trivial for a transacting node to order the signatures, but it is not trivial for a verifying node to validate 500k transactions per day. We need to use the resources of a verifying node as efficiently as possible.