Why do some Christians observe "the covering" and some do not?

Difference of opinion on the teaching in I Corinthians 11:2-16

I Corinthians 11: 2-16 Women praying and prophesying with heads uncovered

1. Praise for having kept the traditions delivered to them (2)

2. A reminder concerning the proper line of authority (3)

3. Concerning praying and prophesying (4-5a)
        a. Every man who does so with head covered dishonors his head (Christ)
        b. Every woman who does so with head uncovered dishonors her head (man)

4. A woman praying or prophesying uncovered would make her appear as one shorn or shaved (5a)
        a. If a woman is not covered, let her be shorn (6a)
        b. If to be shorn or shaved is shameful, let her be covered (6b)

5. It is proper for a man not to cover his head (7-9)
        a. Man is the image and glory of God, while woman is the glory of man (7)
        b. Man did not come from woman, nor was he created for woman (8-9)

6. It is appropriate for a woman to have a symbol of authority on her head,
        a. It is because of angels (10)

7. Men and women are not independent of each other (11-12)
        a. Especially in the Lord (11)
        b. For as the woman is from the man, so the man is through the woman (12a)
        c. And all things are from God (12b)

8. Judge this matter for yourselves (13-15)
        a. Is it proper for a woman to pray to God with uncovered head? (13)
        b. Does not even nature teach you? (14-15)
            1) That long hair on a man is a dishonor to him? (14)
            2) That long hair on a woman is a glory to her, and provides a covering? (15)

9. If anyone is contentious about this matter... (16)
        a. We have no such custom
            (i.e., this is not an "apostolic tradition")
        b. Nor do the churches of God