A bit of an educated guess but I believe the answer stems from markdown originally targeting terminals, which would line wrap but not word wrap. So you could get ugly output like this:

    hello, wor
    ld!

(albeit this is assuming the terminal is only 10 characters wide but I hope you get the point)

So it used to be common for developers and sysadmins to manually wrap to 80 characters (a habit I still regularly catch myself doing even now). Obviously with GUI readers and variable-width characters, you wouldn't want that 80 character manual wrap honoured.