I really cannot answer that, but I can speculate.
I think that it is just a continuation of an old inherited policy (Rome is very slow to reverse itself). If the Church in England had not separated when it did, it is presumed that the liturgy in England would have evolved into the modern Latin practice. This is in fact what happened elsewhere in Europe. The local variations were almost all suppressed subsequent to the Council of Trent, I believe.
The Sarum rite could not be suppressed because it was in schism and beyond the reach of the Roman hierarchy at the time.
So basically I think (I speculate, rather) the reluctance on the part of Rome to grant a permanent place to the Sarum Rite is a holdover from the earlier Tridentine policy that there should be only one Rite in the church, or at least the Latin West, and there has been some reluctance all around to think differently after all this time.
It should be remembered that for a long time, uniformity of practice was a highly valued quality and it took on a lot of symbolic value. Organic variations in the liturgy were not appreciated as they are today.
In our day diversity has a symbolic value as well. Thus, modern Catholics take immense pride in the large variety of liturgical rites in the church. To many it symbolizes the universality and antiquity of the church and gives apologists bragging rights. Not very long ago we still had a completely different widespread attitude wherein uniformity of practice was a great strength, representing a homogeneous belief system, and the non-Roman rites were slowly and deliberately edged into conformity with the practices of the Roman church.
For example, the Ambrosian Rite (Milan) and the Mozarabic Rite (Toledo) are two examples of ancient separate communities in the west that still have representation, but they are not truly separate Rites today because of the many centuries of adapting to the Franco-Latin ritual practices. And they are also just about dissolved, having very very few priests and laity who worship in the tradition. Once hundreds of thousands, if not millions of people were familiar with these rites. The people who still today worship according to these rites are technically classified as Latins by the church.
[Likewise in many areas of Europe the Byzantine rite was practiced, having been introduced through the missionary activity of the Greek east, but in these particular areas (where the churches have been continually under Rome) the churches have mostly been converted to the Latin rite. The modern Byzantine Catholic rite in Europe survived because of years in schism from Rome, and therefore unreachable during that time to those who would change their practices.]
We just have to accept that uniformity of practice, as well as belief, was at one time held to be preferable, and the Latin rite was to be preferred. It’s not better or worse than what people think today, just different.
Anyway, the Sarum rite (or Usage

) survived through schism, and some people don’t like to be reminded of that. It can be particularly galling if the rite in question happens to evince a stronger emotional response from Catholics than what they will naturally give to their own church’s liturgical practice.
It is well known that the strongest opponents of the Anglican Use have been the Latin Catholic bishops of England, and the provision to allow the Anglican Use has never yet been applied there. In fact as I understand it (I might be off on this point), Pope John Paul II excluded England from the possibility of instituting the Anglican Use at their request.