I often wonder if there was an implicit desire to funnel more sede-leaning trads to non una cuм chapels to bolster their coffers (knowing that trads are generally more generous with their tithes). I'm only speculating here, it most likely isn't the case.
It is definitely an attempt to make the theoretical into the practical and to make the faithful "choose". It is an attempt (to use a marketing term) to "differentiate" your "Trad Product" from the others. To try to build a "core customer". Once you have a core base, then you try to grow your market. This brings you many things - $, group size, support, popularity, etc.
I hate to be this cynical but the new-sspx (sedevacantism is wrong) does the same thing and so does the indult (those not "under rome" are wrong). In the beginning of Tradition (in the 70s/80s) it was not like this (for the most part). Trads were in survival mode and you when it's life-or-death, you make friends with anyone who can help. Priests were thankful that they had faithful to rent hotel rooms, buy church supplies (candles, missals, etc) and buy buildings for mass. And the faithful were thankful for a valid priest and Mass.
Even Fr Wathen, who theologically disagreed with the sspx (on feeneyism) and on the extreme sedevacantism said and wrote openly to all his laity (paraphrasing) - You must attend Mass on Sunday and Holy Days. No matter what, you owe this to God and obedience to Church law. No matter how much you disagree with this or that Trad priest, if that is your only option for mass, you go there. If you don't, you commit a sin.
Now we have the opposite where the new-sspx and dogmatic sedes are continually implying that to attend a mass at the "other side" is morally wrong. Now if you pin them down, they'll say "No, no, you have to go to mass to a valid priest". But then, next sunday from the pulpit or in some conversation, they continue to bash/criticize the "other side".
Unfortunately, over a period of time, what this does is it de-sensitizes the faithful to the priest's condemnations of truly immoral masses (i.e. in my opinion...indult/new mass). If the priest is constantly "crying wolf" over legitimate Trad priests/masses (and the faithful see right through this stuff), then when the Trad priest rightly condemns the new mass/indult, the laity are more apt to be tempted to say "Oh well, here goes Fr again...condemning some other chapel/priest."
In the Trad world, the "good ol days" were the 70s/80s, before the indult, when the battle lines were clearer, when mass/sacraments were either Traditional or V2, when *most* Trads had a clear enemy and *mostly* were united in the True Faith. Alas, those days are gone.