Why did the Dominican Order, whose rite was celebrated globally prior to the 1960s, not revise its liturgy in according with the Pauline promulgation while Milan, one diocese, created its own Novus Ordo Missae, essentially the Pauline rite with some gratuitous bits of the old tradition tossed in?
The answer is found in the spirit of the times and could be gleaned from Marco's review of the Report on the Bragan Rite for the Congregation for Divine Worship, a study on everything supposedly wrong with Portugal's native rite—its confusing and embarrassing deviations from what everyone else did—and how those things had to be removed in the new Bragan liturgy. Eventually, the bishops of Portugal decided that the new Roman rite was superior and that retaining Bragan idiosyncrasies would be frivolous. Initially, they intended to create a hybrid rite, something like the 1964/5 revised Roman liturgy in anticipation of the full blown reform, but the change never came. Instead they discarded everything wholesale for the Pauline rite.
I have a hard time blaming Ultramontanism for this. The more responsible culprit is what Hull called the "idol of uniformity." The mob mentality that prevailed at Vatican II and continues to bully bishops into line at local episcopal conferences was present in the liturgical reform process. Everyone was "fixing" or "renewing" their rites, so why be the dowdy exception?