I can’t speak about the situation in the United States, but in France there is still a sharp divide between the two camps (diocesan and SSPX) and one of the reasons is that many French bishops refuse to allow the FSSP to operate in their diocese, or to allow their diocesan priests to celebrate the traditional Mass, arguing that there’s “no demand for it”, despite the fact that multiple studies and polls have found that around 30% of French Catholics have expressed their desire to attend it. Because of this it’s very easy for the SSPX to argue that their presence is necessary.
Something else to consider is that in the comparatively conservative United States, there is a tendency to see the SSPX as being on one side, and the diocese, FSSP, and other Ecclesia Dei communities on the other. This isn’t the case in France, where the divide lies more along “traditional vs non-traditional” lines. In other words, attending the FSSP instead of the SSPX often doesn’t result in meaningful integration in diocesan parish life; you’ll still be considered an “integralist”, “radical Catholic”, etc. For example, in 2012 a French bishop, Mgr. Castet, was almost hounded out of office by his diocese for ordaining six FSSP deacons (deemed “ultraconservative radicals”) as priests. The bishop wrote to his outraged priests and faithful but to no avail, and ultimately the French Conference of Catholic Bishops had to intervene to diffuse the situation, and since then Mgr. Castet has refrained from making any more overtures to “Lefebvrists” in order to keep the peace.
Perhaps in the United States there is more reason to hope for the gradual migration of SSPXers to the FSSP, ICKSP, etc., over the next few decades, but in France the battle lines are much more deeply drawn, and so a prelature for the SSPX seems to me a much more efficient means of effecting reconciliation.