This question underlines two important axioms of literary history. First, a century is an artificial division (what if you said “The Titanic and the Ford Mustang were both in the 20th century”), and, second, “Literature reflects sociological and historical changes more often than it affects them”. In this case, Sheridan, a dramatist (and drama is the most reflective of literary genres), is following a change in class status that was the trademark of the “Age of Reason”, a time when Renaissance notions of royalty and privilege were giving way to enterprise, commerce, and accomplishment, a change that Sheridan both applauds and parodies in his plays, which the broader-class audience (broader than Restoration audiences) appreciated. Blake, on the other hand, at the close of the 18th and beginning of the 19th century, was at the forefront of Romanticism, a return to Nature, a re-examination of Man’s relation to the world, best expressed in the poetic genre, a personal voice and perspective. So, the different genres, the different ends of the (artificial) period, and the different views of society account for the admittedly wide gap between Sheridan and Blake. There is also a very real fin-de-siecle effect; as one century closes and another one opens, society anticipates sea-changes that are often self-fulfilling prophecies.