What happened to the flamines minores?

Question

From the early Roman kings on^*, fifteen gods were each given an officially appointed priest, the flamen, to take care of its cult in Rome.

The three main priests (flamines maiores) were in charge of the cult of Jupiter, Mars and Quirinus, while 12 others (flamines minores) were in charge of the cult of Ceres, Vulcan, Flora, Pomona, Carmentis, Falacer, Furrina, Palatua, Portunes, Volturnus, and two others, unknown today.

We know this list primarily from Varro in De lingua latina who, when he wrote it (late Republic), already considered most of those gods to be obscure. So obscure that most of them are widely unknown today (two of which we even forgot the name of, apparently).

Hence my question: how and why did those (fairly numerous) gods whose cult was actively maintained by appointed officials fall into oblivion during the Roman Republic?

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_{^* According to Livy, Numa Pompilius (second king of Rome) founded this system of flamines.}

Answer 1

In Roman Gods: A Conceptual Approach, Michael Lipka offers the following explanation:

Under the Empire, the focal force of the traditional foci of the traditional Republican cults disappeared. One indication of this disappearance is the well-known scarcity of references to the traditional flamines in imperial sources. Even if we grant that these flamines, especially the flamines minores, may have been indifferent towards mentioning their priesthood in the inscriptions, or may rather have been interested in mentioning it in a different guise (because the flaminates themselves had no longer 'communicable prestigious potential'), this would only serve to prove that they had ceased to be significant personal foci of the deity. This may have happened because the cult itself was in decline, or because the traditional focalization of the cult through the flamines had been abandoned or shifted elsewhere. Those flamines who were attested, may well owe their attestation to a certain popularity of the feriae of the god they represented, but statistics are too limited for further conclusions, although it is a fair guess to say that some flaminates existed until at least the beginning of the second century A.D.

This indicates that the flamines had fallen by an even greater extent during the time of the Roman Empire, after the Republic. Some of the reasoning expressed here could be applied during the Republican period.

Wikipedia has another explanation (uncited):

The flamines minores seem mostly connected to agriculture or local cults. The change to an urban way of life may explain why these deities lost their importance or fell into oblivion.

One more explanation is given in A commentary on Cicero's oration De haruspicum responso, by John O. Lenaghan:

It is also quite probable that all or many of the posts of the flamines minores were vacant at the end of the Republic, simply because they involved too many ritual observances and sanctions to be worthwhile.

This last theory seems to be one of the more sound.