At a guess, it’s because the function of preserving file dates and times is more likely than setting the port to something other than the default, so it gets the lowercase character, whereas ssh doesn’t do anything with files so the port option gets the lowercase character.

The inconsistency is annoying though. I wonder if they could make ssh’s -p option case insensitive so -P works across the board. (Maybe -P is reserved for some unknown future purpose?)

A work-around would be introducing long options and having --port be the option’s long name across all the commands, but then, that comes with its own problems.