If Persona was really of use to the Internet community at large, I think we would've seen it be rapidly adopted, regardless of how much "marketing" Mozilla engaged in.

While it might be nice to have from an ideological standpoint, in practice it just isn't of much value to enough people. This may not be pleasant to admit, but I do think that it's the reality.

Compare it to Firefox. Firefox proved to be a product that was useful to a lot of people, and it rapidly gained a comparatively large share of the market quite quickly.

Persona has been the opposite of that. Its adoption never took off, and there's nothing to suggest that it ever will. It just wouldn't make sense for Mozilla to waste more resources on a dead-end effort, especially when those resources could be put toward improving a product like Firefox, which is actually used by millions of people today.