Almost every Vedic power that survived into the later tradition did so by acquiring a person. Rudra became Shiva. A river became Ganga, with a face, a mount, a descent from heaven and a temple. The Apas did none of this. They stayed a plural, they were never named individually, and no story was ever attached to them. In a mythology that personifies with enthusiasm, that is worth noticing, and it is not because they were unimportant.
The likeliest reason is that they were never allowed to stop being the thing itself. Ganga is a particular river, and a particular river can be given a biography without disturbing what water does. The Apas are water, all of it, in the pot and in the sea and in the rain and in the body, and the moment they became a lady with a mount they would have stopped covering the case the priest actually needed covered, which is the water in front of him. So they remained grammatically plural and religiously general.
The result is a strange kind of survival. The least personified members of the Vedic pantheon are the ones whose cult never stopped for a day. Sipping water before a rite, bathing before worship, the offering of water in the hollow of the hands to gods and to ancestors, immersion at a river, and the water poured at the end of a gift are continuous, ordinary and universal, and every one of them descends directly from what these hymns are doing. The jyotisha tradition, giving them the twentieth mansion, is the only place they are treated as characters at all, and even there they arrive as a plural.