I think that this becomes one of the most critical elements in Wordsworth's poem and how it embraces his understanding of the Romantic philosophy. For Wordsworth, the exaltation of self becomes vitally important. The experience is only as good as to its resonating in the mind of the individual. In this light, memory is more effective than immediate experience for Wordsworth because memory remains subjective, something that exists in the individual. If he were to place a greater value on the experience, than the individual is not as important because the external experience is something where primacy is established. Yet, in being consistent with the Romantic philosophy where the individual subjectivity and its experience is only as valuable as how it is reflected upon in the mind, Wordsworth validates the memory of experience more vital than the experience itself.
Consider another element at play here. The experience is a moment in time, a sensation in an instant. It is bound by time, a scientific concept. Yet, the memory supersedes time, as it is eternal. The memory and the subjective are what enable the individual escape time, a scientific concept. It is in this, in the embrace of something that is outside the rational and scientific, that Wordsworth places primacy on the memory and the subjective, if nothing else as a way to offset the crushing perception of rationality and empiricism that seeks to reduce the individual into atomized entities. In vaulting memory over experience, Wordsworth is able to find yet another way in which he can challenge the rational and scientific Status Quo within which he finds himself.