CRUSOE was not intended as a work of popular fiction in the sense that we use the term today. Though I confess that I have not read Defoe's entire novel, its deeper themes mark it as an ancestor of the form Voltaire later called the "conte philosophique," which he Voltaire coined in opposition to simple "popular tales." This online journal includes an essay by one Peter Leithart, who asserts that "Robinson Crusoe is a novel that continually threatens to collapse into allegory."
CRUSOE was not the first work of "elite culture" ever to become popular with the masses. We know from Ben Jonson's remarks that the audience he shared with William Shakespeare embraced the "mouldy tale" of PERICLES PRINCE OF TYRE, though the folkloric feel of the play alienated many later generations. But I perceive that CRUSOE, writing to a post-literate populace-- that is, one that had become dominantly literate and accepted all the customs of literacy-- validated the nature of the visceral and the popular in CRUSOE in a way that could not have happened with PERICLES or any similar work.
In the opening chapter of the literary survey LOVE AND DEATH IN THE AMERICAN NOVEL, critic Leslie Fiedler suggests some of the reasons that this may have come about. Fiedler deems the eighteenth century to be the period of a "continuing, complex event" he calls "the Break-Through," in which many if not all of the old societal beliefs began to undergo a systematic reversal.
Whatever has been suspect, outcast and denied is postulated as the source of good... in a matter of months, Don Juan, enemy of Heaven and the family, has been transformed from villain to hero, and before the process is finished, audiences have learned to weep for Shylock rather than laugh him from the stage. The legendary rebels and outcasts, Prometheus and Cain, Judas and the Wandering Jew, Faust and Lucifer himself are one by one redeemed.
The same principle holds for that ancestor of the horror-genre, the Gothic:
..."gothic" passes from a term of contempt to one of description and then of praise...
Fiedler, although he would champion popular fiction in many of his later works, does not treat popular fiction as a separate subject in LOVE AND DEATH, as this work is concerned largely with the novels that became canonical literary works. But I assert that, in addition to the developments Fiedler cites for the Break-Through, another such turnabout is one in which fiction aimed at the masses began to be valued. Admittedly, the next three centuries had no shortage of highbrow critics inveighing against the merely popular, one example of which can be read here.
ROBINSON CRUSOE, perhaps as much or more than THE CASTLE OF OTRANTO, may have played a key role in fomenting the rise of "popular tales," even if neither Defoe nor Walpole intended to bring about such a revolution in literary taste. Significantly, CRUSOE is the first novel of the 18th century to become popular enough to generate two sequels, as well as engendering its own eponymous genre, the "robinsonade." Predictably enough, many of the works listed as followers in this generic tradition-- particularly the best-known, the 1812 SWISS FAMILY ROBINSON-- have little of the "conte philosophique" about them. But this was only fair, given that "elite culture" had a long tradition of borrowing from the pre-literate forms of "popular fiction"-- with the great Bard of Avon fairly well leading the pack in this respect.