 The reason for this, as Tovey remarked, is that the purpose of the Prelude is to generate a sense of expectation leading towards the piano entry, and this must come from the music itself, not just from the title on the top of the page.If a complete sonata form were imposed on the Prelude, then it would take on a life of its own, so that when the piano entry occurs, it would be rather incidental to the overall structure.To express it in another way, in sonata form, the first group of subjects is linked to and generates an expectation of the second group, which would tend to detract attention away from the piano entry – a point that, as Tovey points out, was only grasped by Beethoven rather belatedly.