Why is Daniel Defoe said to have written the first English novel with Robinson Crusoe?
Prior to the publication of Robinson Crusoe, novel-length works in English were rare. Most were translations of texts from other languages, collections of essays, or rewritten works from years before. Robinson Crusoe was unique in four ways: it was entirely original; it was entirely fictional; it was very long; it was a tremendous success.
As an entirely original and fictional work, Crusoe was perhaps only preceded by The Pilgrim's Progress, a work that drew on Christian tradition. Crusoe was not based on any specific event or person, and the events inside were entirely invented by Daniel Defoe.
As a long work, Crusoe overshadowed most written books at the time; even other claimants to the position of "first English novel" are often described as "short novels," making Crusoe the first proper novel as defined today.
As a tremendous success, Crusoe inspired numerous copycat works (the most famous of which is The Swiss Family Robinson) and opened the door for writers to experiment with new ideas and new formats for stories. Publishers also started to take more chances with their books in the hopes of a similar payoff; writers who may have given up now had new reason to submit their long works.