Apologies, this is going to be heavy on literary theory and light on history. As a note to begin, it's worth mentioning that Robinson Crusoe is only typically considered one of the first English language novels - see an answer I posted here for a more detailed discussion of the development of the novel.
As to the question at hand, it's a case more of development of literary theory than of history. Crusoe's distinction as one of, if not the, first English-language novel is attributable, IMO, to the work of Ian Watt, specifically his book The Rise of the Novel (1957, I have never encountered his ideas in an earlier work), in which he calls Robinson Crusoe the first modern novel (an argument which tends to be cited as a reasonable scholarly consensus, but you will find earlier works called novels, though typically with a caveat. An example of this is Aphra Behn's Love-Letters Between a Nobleman and His Sister, which predates Crusoe by 35 years and is typically referred to as an epistolary novel or simply an epistolary, as its chief storytelling conceit is letters). This status is based on three different attributes: realism, the individual or "inner life", and (this is my assertion rather than Watt's) an issue of plot versus story.
The realism is seen as a departure from the "romances" of the late middle ages and Renaissance. These tended to be fairly fantastic and/or poetic (consider examples such as Sir Gawain and the Green Knight or The Canterbury Tales). This had been done before with novellas (known as novelles at times prior to Crusoe, but today these are considered too short to be proper novels. Nevertheless, Crusoe opens the novel with an insistence that he has presented everything in the story as it happens and follows with an entirely plausible story: a young merchant is marooned and island and, through his resourcefulness and thanks to God's providence, finds his way back home, a man changed by his experiences. Robinson Crusoe is intended to be read as something that really could have happened - not the first work to do so, roman a clefs were popular long before Crusoe (stories about real life/events disguised as fiction, e.g. On the Road, All Quiet on the Western Front, Animal Farm to a degree), but nonetheless, its realism is seen as a departure from what came before.
The role of the individual and the "inner life" are, IMO, the biggest divergences. I don't know if you've read Robinson Crusoe, but it can be downright ponderous at times. Within Dafoe's lifetime, abridged versions were put out that claimed to deliver all of the action and story with as little of the slow-paced reflection as possible. The amount of time Crusoe spends reflecting on his position, the meaning of it, his relationship to Friday, the evil or morality of the island's cannibals, his faith, etc. is fairly atypical of other works at the time. Today we take this sort of thing for granted. The two most typically used narrative perspectives in novels now are first-person and omniscient third-person. We know how at least one character is thinking, feeling, and reacting - we know them in a way. This is why bad characterization is so readily called out today - in a medium where one of the defining characteristics is how thoroughly one can be immersed in a character's life and experience, having an unlikable or poorly written character is the cardinal sin. This emphasis on the inner life is an acknowledgment of individualism that is, again, not particularly common. Individualism as we think about it now was a relatively new idea. Prior to the advent of individualism, there wasn't much concern for the inner life in literature, particularly in fiction. Stories are concerned with what he did, what he said, where he went, not what he thought or felt. He thought and felt the same thing as everybody else.
This is where I connect the idea that a big separator for Robinson Crusoe is its emphasis on plot over story. I use these words in the way literary theorist E.M. Forster describes them, and I certainly can't put it any better than he did:
We have defined a story as a narrative of events arranged in their time-sequence. A plot is also a narrative of events, the emphasis falling on causality. ‘The king died and then the queen died,’ is a story. ‘The king died, and then the queen died of grief,’ is a plot. The time sequence is preserved, but the sense of causality overshadows it. Consider the death of the queen. If it is in a story we say ‘and then?’ If it is in a plot we ask ‘why?’
Using Forster's elegant explanation here, you can see why the inner life is essential to the development of plot superseding story. It is necessarily intertwined with the why question. Reduced to a story, Robinson Crusoe is essentially "A young man set sail to find his fortunes but misfortune marooned him on an island. He learned to survive there and was eventually saved by his countrymen. He returned home and settled into the happy, wealthy life he originally left for." It's utterly meaningless (except as a capitalist fable, as some argue Robinson Crusoe is). It's the addition of Crusoe's inner life that makes us stop and ask why, and that question is at the heart of the distinction.
Sources:
And a healthy amount of consultation of lecture notes re: Robinson Crusoe's themes and contemporary significance.
/u/The1Man offers a far better description of the early English novel than I can, but I'd like to point out one particular example from Arabic that I think might be useful in comparison.
Although there are many possibilities for "earliest novel" Hayy ibn Yaqzan written in Arabic by ibn Tufail in the 12th century is a decent contender. We don't know for sure whether Defoe read Hayy ibn Yaqzan but it was popular in London at the time, and it seems quite likely that he was at least familiar with it. Like Crusoe the novel is about a man stranded on a deserted island, and, as /u/The1Man points out, is largely focused on an examination of the self and the inner life.
Wikipedia is obviously a poor source for this subreddit but this article is well sourced and is reflective of what I've read elsewhere: http://en.wikipedia.org/wiki/Hayy_ibn_Yaqdhan
The transfer of Arabic literary genres to Spain was a key part of this development, so it's probably not a coincidence that this was written in Spain. Other Arabic contributions to European literature via spain can be found in a comparison of the maqamat (particularly of Hamadhani) with the picaresque.