Leonn, Traditional priests are in a situation (necessity/emergency) brought about by the crisis in the Church, where it is, generally speaking, morally impossible for them to be under the direct authority of their legitimate superiors, because those in authority are using that authority, not at the service of the Faith, but to destroy faith and morals. Bishop Williamson talks about this split between truth and authority again in his recent Eleison Comments.
This is the concern that the Resistance has with the SSPX, that it is now willing to place itself under this corrupt authority which is destroying the Church, and accept jurisdiction from them. This will place them in a position which they themselves had always taught will lead them to slowly but surely lose the faith, "it would mean our death" they repeatedly said.
Obedience is at the service of the Faith, and the Laws of the Church are for the salvation of souls: suprema lex, salus animarum, the supreme law of the Church is the salvation of souls.
"Canon Law likewise is directed to the salvation of souls; and the purpose of all its regulations and laws is that men may live and die in the holiness given them by the grace of God" - Pope Pius XII, Address to the clerical students of Rome, June 24, 1939
You may find the following explanation useful. It is from the study by Fr Francois Pivert "Schism or Not" which was published after the 1988 Consecrations:
Canonical considerations excerpted by Fr Patrice Laroche SSPX from a study by Dr Georg May, President of the Seminary of Canon Law at the University of Mainz, entitled 'Legitimate Defence, Resistance, Necessity', drawn up in 1984:
"The 1917 Code of Canon Law spoke of necessity in Canon 2205.2 and 3; the 1983 Code of Canon Law deals with it in Canon 1324.4 and 1324.1 and 5... it is clear from the context that necessity is a state where goods necessary for life are put in danger in such a way that to come out of this state the violation of certain laws is inevitable. The Code recognises necessity as a circuмstance which exempts from all penalties in case of violation of the law (1324.4)... no latae sententiae penalty can be incurred by anyone who has acted in this circuмstance (1324.3)... In the Church, as in civil society, it is conceivable that there arrive a state of necessity or emergency which cannot be surmounted by the observance of positive law. Such a situation exists in the Church when the endurance, order or activity of the Church are threatened or harmed in a considerable manner. This threat can bear principally on ecclesiastical teaching, the liturgy and discipline. A state of necessity justifies the law of necessity. The law of necessity in the Church is the sum total of juridical rules which apply in case of a menace to the perpetuity or activity of the Church... The law of necessity also includes the positive authorisation to take measures, launch initiatives, create organisms which are necessary so that the Church can continue its mission of preaching the divine truth and dispensing the grace of God. The law of necessity uniquely justifies the measures which are necessary for a restoration of functions in the Church... In a situation of necessity the pastors of the Church can take extraordinary measures to protect or reestablish the activity of the Church. If an organ does not carry out its necessary or indispensable functions, the other organs have the duty and the right to use the power they have in the Church, so that the life of the Church is guaranteed and its end attained. If the authorities of the Church refuse this, the responsibility of other members of the Church increases, but also their juridical competence."