In the explanatory letter accompanying the motu proprio, the Holy Father talks about the reason for the surprising tenacity with which some of the faithful held onto the Latin Rite Mass:
This occurred above all because in many places celebrations were not faithful to the prescriptions of the new Missal, but the latter actually was understood as authorizing or even requiring creativity, which frequently led to deformations of the liturgy which were hard to bear.This is the crux of the issue, I think. When "everything changed" in the mid-1960's, suddenly so much of what had been taught was seemingly completely thrown out. Much of the "creativity" that occurred with the "New Mass" was pointedly against Catholic Church teaching, and, we are now paying the price for generations of the faithful (and now, many priests, sadly) who have grown up with this nebulous non-teaching education, both formally (in religious education classes of various stripes) as well as by osmosis (in what they saw going on around them in the Mass.)
The Holy Father responds to fears that the "freeing" of the Latin Mass will cause a split in parishes between those preferring it and those preferring the Novus Ordo--Mass in the Common Language, (also known as the Mass of Pope Paul VI).
The most sure guarantee that the Missal of Paul VI can unite parish communities and be loved by them consists in its being celebrated with great reverence in harmony with the liturgical directives. This will bring out the spiritual richness and the theological depth of this Missal.In other words, if the transition to the Mass (in English) had gone the way Vatican II documents prescribed and assumed it would, we might not have had this outcry for the so-called "Old Mass" (also known as Tridentine or Latin Mass) in the first place. Further, those preferring to hear Mass in English would not feel so threatened by those preferring to hear the Mass in Latin (and vice versa) because both versions of the Roman Rite (and the Pope stresses they are both the same rite) would be said reverently and be recognizable as The (same) Mass.
In the 40 years since Vatican II, we have lost a sense of what The Mass is. In the resulting vacuum, people have substituted other meanings for The Mass (which is now more often than not, called anything but "Mass" these days). With the Motu Proprio, the Holy Father is bringing back the real meaning of the Mass.