Yeah keeping Joanna McCoy's daughter and the Chekov love angle would have made sense. I wonder why they decided they had to drop Joanna entirely.
I read somewhere that somebody thought that giving McCoy a grown daughter would make him seem too "old." Not sure if that was the deciding factor.
Also, remember that "divorce" was a controversial topic back then, which was known to make studio executives nervous. As late as 1970, CBS wouldn't allow Mary Tyler Moore to play a divorcee on The Mary Tyler Moore Show.
Which could be another reason McCoy's backstory never actually made it onto the screen--until 2009, that is!