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?
* According to Livy, Numa Pompilius (second king of Rome) founded this system of flamines.