There is a myth which is endlessly repeated ... I groan every time I read it ... about the liturgical reforms of S Pius V. It goes like this:
(1) He wished to standardise and centralise. So he ordered everybody to use his new edition of the Roman Missal (but he did permit those with rites more than 200 years old to keep them).
This is pretty well the opposite of what his legislation ordered. He:
(2) Ordered those with such old rites to keep them. But, if they positively wanted to adopt his new edition instead, he permitted them to adopt it AS LONG AS THE BISHOP AND THE UNANIMOUS CHAPTER WERE IN AGREEMENT.
If you don't want to believe me, I suggest you read the actual TEXT of Quo primum yourself and find out. DO NOT READ SOMEBODY'S SUMMARY OF THE BULL, BECAUSE THAT WILL (almost certainly) JUST TELL YOU THE MYTH.
If you want more detail, with a fuller account of the liturgical context in the middle years of the sixteenth century, you will find a much longer explanation on my blog, 6 December 2014.
I sha'n't enable comments which show no signs of the writer having read the bull or of reading my piece from last December and simply splutter at me; or disdainful comments which tell me that I'm just splitting hairs. If you can't see the immensely profound difference between (1) and (2), it's not worth having a discussion.