It has happened in the case of some of the Vedic inherited gods that exactly in proportion as their popularity decreased their greatness increased; that is to say, as they became more vague and less individual to the folk they were expanded into wider circles of relationship by the theosophist, and absorbed other gods' majesty.[89] Varuna is no longer a popular god in the Rig Veda.

He is already a god of speculation, only the speculation did not go far enough to suit the later seers of Indra-Savitar-hood.

Most certainly his worship, when compared in popularity with that of Agni and Indra, is unequal.

But this is because he is too remote to be popular.
What made the popular gods was a union of near physical force to please the vulgar, with philosophical mysticism to please the priest, and Indra and Agni fulfilled the conditions, while awful, but distant, Varuna did not.
In stating that the great hymn to Varuna is not typical of the earliest stage of religious belief among the Vedic Aryans, we should add one word in explanation.

Varuna's traits, as shown in other parts of the Rig Veda, are so persistent that they must be characteristic of his original function.