Because Christianity is not libertarian, and we are not to hate our brethren in our hearts by suffering sin upon them, without loving correction, reproof, etc., which we should also be thankful to receive when we, ourselves, are in error. The bringing out historical opinions is to show that they believed the Scriptures were not silent on these matters, and that we are subject to God Who commands how He will be sanctified, and would have us to cast aside our inventions.I've also never really understood dragging out historical opinions of people in different cultures/historical contexts. Since no one forces us to engage in "holy-days" what's the point?