Mackenzie claims that parity was explicit concern for selecting 7 bit code for ASCII. He cites X3.2 subcommittee, although does not provide any references which document exactly, but considering that he was member of those committees (as far as I can tell) I would put some weight to his word.

https://hcs64.com/files/Mackenzie%20-%20Coded%20Character%20... sections 13.6 and 13.7