Part of the inadvertent legacy of the works of Beethoven is that from one piece to the next, a composer began to be expected (and expected of themselves) that the following piece was to break new ground, i.e. the composer was to say something essentially new about both form and content.
Until then, we have composers writing in a similar vein, the better ones developing their vocabulary or honing the finesse of what they wrote, over the progression of hundreds of symphonies, forty symphonies, etc. Along comes Beethoven, practically re-inventing himself and the form and content of that medium from one piece to the next, with a total of only nine in his lifetime.
That tendency of making an entirely new and fresh musical statement held, and increased to such a degree that by the 1950's or so, even undergraduate composition students were being told their next work should be an entirely different investigation into music than the last -- before they even had the more well-advised practice of writing and writing until they found "their own voice."
That accounts, in one manner of notions in the air at the time, for why those more notable composers made fewer sonatas, and fewer symphonies. Too, there was a very strong interest in freer more rhapsodic forms, both small and large, to which they devoted a lot of their time.
Mozart, 20 + piano concerti. Beethoven, 5. Brahms 2. Prokofiev, 5. Shostakovich, 2
etc. etc.