The funny thing is that I remember a talk from Bishop Fellay from years back suggesting that he saw no reason to do this.
It's probably 10 years old but I remember him relating how he talked with the Vatican about mutable truth and how their beliefs were incompatible and he concluded "OK, we wait".
Why now? I don't get it.
It seems to me that this all went into high gear when the SSPX had that massive pilgrimage to Rome about 10 years or so ago. I think the powers that be in the Vatican, especially Benedict, seeing their impressive numbers, recognized that there was a need to bring them in.
During the negotiations and discussions, in my opinion, I think the SSPX got crushed by the Conciliar sect. Aside from all of the discussions on what form the SSPX would take if it is integrated, the other and more important part of the discussions revolved around doctrine. This is where the SSPX theological position cannot stand up to scrutiny.
While the SSPX does defend the Faith on some points, religious liberty, ecumenism, etc., they fail miserably when it comes to explaining the crisis in terms of correct ecclesiology, and the binding power and safety of the ordinary magisterium. I am sure that sect's theologians pressed these points during the talks.
I could see them (the conciliar sect) showing the SSPX delegation the following:
1. A rite of mass approved by the Pope, with its authorized translations and usages, could not contain doctrinal error, lead to impiety, or in any way be evil.
2. The Pope cannot teach the universal Church heresy or doctrinal error, even in his non-infallible doctrinal teaching.
3. There could never be a reason to resist or disobey the Pope by rejecting his authoritative teaching. It is not even permitted to keep just a respectful silence, full internal assent must be given.
All of these points are provable and I cannot see how the SSPX when pushed on these matters could even begin to answer their counterparts. After all, the Pope is Tradition, as Pius IX taught, so he is the judge of what is Traditional or not, not private individuals.
The underlying foundation of the SSPX position is an implicit belief that that the Shepherds are not actually the rulers of the sheep. The subjects of the Pope can judge his teaching and law: authoritative teaching as given through encyclicals, and other teaching documents, etc., along with canon law, sacramental rites, canonizations, etc. The position logically leads to an inversion of the structure established by our Lord for how the Church functions in regards to the teaching and governing power given to the Pope.
Now, the SSPX will deny this, they will state, "when we have an orthodox Pope who upholds Tradition we will believe and obey him." A statement like this is actually implicitly heretical, as it implies two errors against the Faith: that a Pope can destroy or undermine sacred Tradition and Teaching in the first place, and secondly that it is for the laity to judge the Pope's teaching and law to determine what is to be received and accepted and what will be rejected.
When the sedevacantists challenged the SSPX on these points, we were for the most part ignored. Now that they are in serious discussions with the sect's theologians, they will not be able to evade these serious questions about the inherent proble,s and weaknesses in their position.
The SSPX inherently is built on a house of sand, and it is falling down. Fr. Barbara repeatedly tried to warn and admonish Archbishop Lefebvre to no avail. The entire concept of forming a seminary as a response to this crisis is in my view gravely flawed.
The only course of action that the hierarchy had was their God-given authority to publicly denounce the heretic Paul VI and call for an imperfect council to elect a Pope. Lefebvre did not do this, he did not respond correctly with the use of his office, and chose to resist Paul VI while accepting him as Pope, and began training and illicitly ordaining mission-less priests as his response.
Now, decades later, we are witnessing the fruits of this tragic error in judgment.