There may be a combination of real and imagined barriers to greater numbers of pilgrims:
Real barriers include the fact that the Camino is long. The climate is hot in summer (e.g. 45º in Cordoba) and cold in winter (e.g. snow in Granada). The terrain offers a few "typical" Camino challenges (e.g. some roadside walking, some stretches with little shade) and some "special" challenges (e.g. some stages with no water or villages, a scramble up a rocky hill, and dry riverbeds that tire you out)..
Imagined barriers include the reputation that only very experienced or tough pilgrims can handle it. Some guidebooks say that the Mozarabe is "Unsuitable for first time pilgrims," or "Lacks facilities," or "Demands Spanish language ability." I think that this puts people off a Camino that has actually gotten considerably easier in the last few years.
Over time, I think the reptation of the Mozarabe will grow and more people will get interested, despite the challenges. The associations have built up a good network of albergues. They have marked the route very well with yellow arrows. And they are exceptionally supportive and welcoming - especially the association in Almeria.
Comparison is difficult. I don't know Norte, Primitivo, and Salvador. Compared to VDLP, the terrain of the Mozarabe is a bit more challenging with higher peaks to scale. The network of accommodation is somewhat better. The number of pilgrims is lower. The history and cultural heritage is stunning - roman ruins, moorish architecture, prehistoric archeology, and castles. The natural scenery is not better or worse, just different - desert landscapes, Sierra Nevada mountains, Wild West badlands, and wide open prairie lands.
The Almeria Association reports that it issued 1,000 credentials in the first half of this year. If you look at the recent blogs, you'll see more photographs of pilgrim encounters than you might have seen a few years ago. It will never be a crowded path, but I think it might be in the Goldilocks zone for people who want the comfort of a well-supported Camino but the tranquility of a less-discovered route.