The reason is because, as Father Zuhlsdorf has often pointed out, the Pope’s use of the terms “Ordinary Form” and “Extraordinary Form” deals with a juridical question only:
[quote=Father Zuhlsdorf]…by stating in *Summorum Pontificum *that there are two uses of one Roman Rite, Benedict has made a juridical distinction. This is critical to understand how Benedict derestricted the older form of Mass so elegantly.
By saying that, considered juridically, there is only one Rite in two uses, Benedict eliminated the need to grant special faculties (canonical “permission to say Mass” coming from proper authority) to say the older Mass. If a faculty can be given, it can be withheld or withdrawn. By saying that there is just one rite, juridically, Benedict has seen to it that if a priest has the faculty to say Mass in the Roman Rite at all, then he has the faculty to say either Mass, the older or newer form or use. This is a juridical distinction.
Benedict is not, I believe, saying that there is no longer a question of whether or not the Novus Ordo is, considered historically, liturgically, theologically, etc., a different Rite. This was an elegant *juridical *solution.
I think the question remains open about whether or not the Novus Ordo is really a different rite.
Frankly, I think it probably is. I think the changes made were different enough to constitute it as a different rite. I frankly think that that is what Benedict XVI thinks too, based on what I have read and also knowing the great esteem and harmony he has with Klaus Gamber.
A lot more study of this needs to be done and I sincerely think the door is still open for that study. The need is sure there!
That said, I thank His Holiness for the elegant juridical solution in *Summorum Pontificum *of considering there to be one Roman Rite, juridically considered.