Rasana's tanmatra, rasa, is the fourth link in the chain that ends in the five gross elements, and the element it feeds is apas, water, traditionally described as having taste for its sense and flavour, rasa, for its subtle quality, drawn from the same body of Samkhya literature this entry draws from. Rasana is the organ-side half of that pairing: the perceptual faculty whose sole object, flavour, is also the quality water specifically contributes once enough tanmatras have accumulated for it to condense out of the evolutionary sequence.
The correspondence is not merely thematic. Samkhya's account of grossening is cumulative, each gross element carrying the tanmatra qualities of every element before it in the sequence plus one of its own, so water carries sound, touch and form in addition to its own distinctive taste, while fire, one step earlier, carries sound, touch and form but not yet taste. This is why taste is described as first becoming perceptible at the level of water rather than at the level of fire or air: it is literally the newest quality water's position in the sequence contributes.
Rasana's place fourth among the five Jnanendriyas, and its narrow restriction to rasa alone, both follow directly from this structure. An organ built to register a quality that only becomes available at the fourth stage of grossening has no occasion to register anything earlier or later in the sequence, which is exactly the strict one-to-one discipline Samkhya maintains across all five organ-tanmatra-element chains.