That’s an interesting question, thanks for asking it.
I think Holy is left out because we believe that God is holy and don’t want that as an attribute to a living human being, as we’re not holy by definition. Although, some of our works could be considered holy I suppose?
I think Saintly is left out because it’s just not a common phrase in English. I don’t know if there’s any Religious reason that Evangelicals don’t call people “saintly”. I think it’s just not a common term, so it’s not commonly used; that’s probably about it.
I think that godly is used because Christians want to emulate Christ as closely as possible. I think when someone says someone is godly they are speaking of the works of the person, and not the person themselves. Evangelicals are usually pretty afraid at offending God by making anything or anyone else out to be a ‘god’ in any way. It makes us all uncomfortable. So I think saying a person is ‘godly’ is saying that their works emulate that of Christ. Christ is God, therefore feeding the poor is a ‘godly’ thing to do.
This is sort of speculation on my part though based on my experience. Again I insist that Evangelicals are really careful not to make any person/thing out to be God or a god. We even worry that we may make a concept our god, like lust, power or pride.
Hope that helps a bit