It was upheld in at least four documents, one which is official. Saying there is no trial, so it is invalid, is an invalid defense.
The best one is the in the words of John-Paul II himself, the
Motu Priprio:
In itself, this act was one of disobedience to the Roman Pontiff in a very grave matter and of supreme importance for the unity of the church, such as is the ordination of bishops whereby the apostolic succession is sacramentally perpetuated. Hence such disobedience - which implies in practice the rejection of the Roman primacy - constitutes a schismatic act. In performing such an act, notwithstanding the formal canonical warning sent to them by the Cardinal Prefect of the Congregation for Bishops on 17 June last, Mons. Lefebvre and the priests Bernard Fellay, Bernard Tissier de Mallerais, Richard Williamson and Alfonso de Galarreta, have incurred the grave penalty of excommunication envisaged by ecclesiastical law.
There doesn't have to be a trial. An abortionist is excommunicated without a trial, they have been given warnings, and know the act they are performing leads to excommunication. There is no adversarial Law in the Church, as we Americans know it.
The act was fully admitted by the participants, and they were given clear lawful instructions on how to reverse the act. Among them were to hold a Novus Ordo Mass, to show what they mean by the Mass is licit. If it is not licit as a Mass, then the SSPX is not going to be able to be brought back into the fold.
Since is it so subject to local interpretation, could not the smart people of the SSPX figure out how to say the Mass in the manner of reverence they prefer, and yet in the Novus Ordo style?
The real answer is POWER. Many have a great deal of respect as a leading schismatic in the SSPX, that standing would go away if they regularize.
Like I said before, look at some people here, and judge for yourself. I should make it clear, Ultima Ratio, for all the twists he does, and all the unwelcome tweaking I may employ, isn't a bad guy. He comes off as a gentleman, and perhaps he will be able to come to a position where he can return to Catholicism.
Others, like some sedavacanists, think by shouting pedophile, or shiva, or whatever, think that substitutes for an argument. The latter group may never return to the Bosom of the Church.