The topic of "why is the Resistance so small" is a large one, and one short email can't do it justice. So I at least need to write a quick followup (since I don't have time for a book)
Take the group "Catholic" -- the majority are Novus Ordo, some shouldn't even be called Catholic (because they would actually reject the Catholic Faith if someone were to corner them and confront them about Church teaching, and the falseness of Vatican II) and a thin sliver of the whole are loosely traditional.
From that loosely traditional group, you have a bunch who think you need Roman permission to attend a Tridentine Mass -- they depart for the FSSP and other Indult groups.
You are left with Traditional Catholics. How many of them spend much time and energy on "church stuff" outside of getting ready for Mass on Sunday morning, attending Mass, and driving home? I think some Catholics believe that anything more would be "extreme", "overdoing it", being a "holy roller", or "if I'm going to go that far, I should have been a priest". I hate to say it, but only a thin remnant of the Traditional Catholic remnant expend any time or energy in reading, discussing, understanding the position, or really CARING AT ALL about matters of Faith.
You are left with a smaller subset of Integral (7 day a week) Traditional Catholics who not only consider themselves Catholic, but Catholicism would also qualify as a main hobby, insofar as they spend a lot of their free money and/or time on matters related to it (study, discussion, reading, volunteering, etc.) Everyone in this group develops principles (leading to favorite positions, favorite groups) due to their intense INTEREST and CARING about the subject.
But in this group of serious, Integral Trads, you still have large variations in terms of education, intelligence, strength of emotions, personal experience, and accidents of location or chance (I met a great priest with the SSPV...)
Some in this group decide (or "feel") that sedevacantism is the only way.
The rest of the group, getting pretty small by this point, decide with the more cautious, balanced (some would say prudent) Recognize and Resist position.
Now in days of yore, this would be the SSPX. But today, the SSPX is visibly losing it, which necessitates the creation of the Resistance or continuation of the old SSPX position. But how many SSPX attendees have actually left and attached themselves firmly to the Resistance? At my chapel (San Antonio, chapel dating back to 1975, population somewhere between 250 to 300) it's just my family that left -- unless you count 1 additional family that went Pfeifferite. One couple has started coming regularly to the last several (Bp. Zendejas) Masses here, so I'll give them credit at least for supporting the Resistance. There are a few other individuals/families which come to the Resistance chapel "part time", but they all still attend the SSPX.
At least 8 vehicles/famlies have left the San Antonio SSPX chapel to join the Indult, and let me point out this isn't one of those awesome-sermon-giving FSSP priests you may have heard about. No, this is INDULT, said by older priests who aren't very good at it. One of the priests there (no longer there) dabbled in the Charismatic movement as well as the Latin Mass. We're talking about a shared facility, too -- the same building is used for Novus Ordo (with particles -- or Particles -- of Communion in the Hand all over the floor)
When you consider all the obstacles -- the gauntlet -- that a Catholic has to run in order to arrive at the Resistance position, it boggles the mind.
Dimond Brothers
SSPX "obedience", pro-modern Rome, pro-accord propaganda
1962 Missale vs. 1954
Una cuм issue -- to mention the Pope during the Canon or not?
Novus Ordo Watch and other Sede sites
Other dogmatic, schismatic sedevacantists on the Net (Pope Michael, etc.)
Indult groups in every major city "we have to obey the Pope"
Pfeiffer cult
That last item is of particular note: of all those SSPX Catholics who "woke up" to the Crisis in the SSPX, how many of them went with Fr. Pfeiffer? Keep in mind that Fr. Pfeiffer was a Resistance pioneer. The most alert and awake SSPXers would have had no other choice if they wanted to resist the new SSPX orientation back in 2012 or 2013. Now it's true that some or many (most?) of these have since left Fr. Pfeiffer and joined the mainstream, worldwide Resistance under the 4 faithful bishops. But a lot of the principled, especially emotional, ones stuck with Fr. Pfeiffer to the present day. Even those who aren't 100% emotion might be convinced by Fr. Pfeiffer -- cult leaders are often very convincing in person, and that includes Fr. P. Catholics are naturally very hesitant to believe a priest could lie to them. So that also takes a noticeable number away from the main Resistance population.
Long story short, serious minded Catholics are likely to do research, form strong opinions, and many of those opinions are going to conflict and go different directions. It's also hard to give credit to your opponents without implicitly admitting that you might be wrong. If you give credit to Sedevacantists for example, then why? Is it because you want to give credit to those who fail to respect the Pope? Of course not. Or if it's because they are holding the true position, then you would obviously be holding an INCORRECT position, since yours is in conflict with theirs. So when I give any credit/respect to the Sedes, either I'm giving aid to enemies of the Catholic Pope, or I'm admitting I'm wrong. See the problem?
Of course I would offer a TERTIUM DATUR (a third option to solve the dilemma) -- that both sides can be of good will, but the Crisis is so confusing that we can't know for certain who is right. So we respect each other as Catholics even as we disagree with how to deal with (sometimes important) elements of the Crisis.
Matthew