In the stdlib, there is only one use of to_bytes() with sys.byteorder (2 in tests), 16 uses of to_bytes()/from_bytes() with 'little' (22 in tests) and 22 uses with 'big' (33 in tests). So making sys.byteorder the default will help almost nobody, and the advantage of 'big' over 'litte' is so small, that making any of them the default may only help in a half of cases and confuse in the other half.