I’ll tell you why even if it is politically incorrect to do so. There are those on this forum who positively loathe and despise the Traditional or the Extraordinary Form of the Mass and everything associated with it. Quite frankly they hate it with a burning passion. They also loathe the SSPX, primarily due to their devotion to that form of the Mass. They will deliver other arguments for their hatred but the loathing of the Mass itself is almost always at the center of their rationale.
Thus the declaring of the excommunications to be ineffective really tears at them because the SSPX is the group primarily responsible for the Traditional or Extraordinary Form of the Mass even being around today. One of the major reasons that the indult proclaimed by Pope John Paul II was extended in the first place was for their benefit in an attempt to get them back into the Church.
So anything done that benefits the SSPX will be seen as a tacit endorsement of the Traditional or Extraordinary Form of the Mass in their eyes. And they cannot tolerate that, under any circumstances. Especially when you factor in the hatred they have for Archbishop Lefbvre himself. They fear now that his stance, far from being, how do they word it, the desperate acts of a disobedient French Bishop, instead will be seen as a valiant stand to hold on to the Church’s timeless traditions in the face of an onslaught of modernism.
As I said, it was probably politically incorrect to say that, but at times the truth needs to be said.
And lest anyone think that I am a SSPX supporter, I never have been. But I have never been their opponent either. I saw what they did, and I know why they did it. In many ways they have been proven to be correct. They still have some things they now must do, but if they do them, , then I think the whole episode will go down in history as a terrible terrible tragedy that need never have happened in the first place.