Beethoven, Bach, Schubert, etc, etc are well known today. They are obviously extremely skilled. But why do we know of them instead of other composers of their era?
Was it because they were extremely popular at the time? It seems most would have never heard their music. Were they the only composers of that era of note? Were they unusually skilled, and we have a lot of bad composing lying around for other patrons? Were they the only music that survived for some particular reason?
To be blunt, it's the way that we have decided to remember them. Unfortunately in history, there's a tendency to tell the story that you want to tell and conveniently leave out other parts that don't fit the narrative. With music history (which practically was a German creation), that narrative was to prolong a lineage of "geniuses" so that you can draw a line from Bach to the modern day.
There certainly were very popular composers back in the day that we barely even perform today. For example, if you go back into the 1860s and 1870s, Joachim Raff was one of the most popular composers. His symphonies were grand successes. But shortly after his death in 1882, he practically fell completely out of the public's mind. His surviving family had hoped to live off of the royalties of his works, but essentially fell into poverty. (Edward MacDowell, an American composer who studied with Raff ended up helping that family out. See Bomberger, E. D. (1997). Edward MacDowell, Arthur P. Schmidt, and the Shakespeare Overtures of Joachim Raff: A Case Study in Nineteenth-Century Music Publishing. Notes 54, no. 1).
Anyway, I digress. If you read newspapers from the 19th century, there are likely performances by many composers that we haven't even heard of today. The composers that we know today from this era (Schubert, Schumann, Brahms, Wagner, etc.) are highlighted because they were seen as "geniuses" and "innovators." In rare cases, they just mastered the art (for example, Mendelssohn today isn't seen as an innovator except in his younger days, but he is generally regarded highly as a master of counterpoint and partially responsible for bringing Bach back into the public.)
Particular composers, historians, and theorists saw themselves as continuing the tradition. For example, Arnold Schoenberg saw his music as the next logical step from Brahms. Heinrich Schenker developed his theories of music to explain why these great composers were masters (his list of twelve master composers was almost exclusively German with the exception of Dominico Scarlatti [for some reason] and Frederich Chopin).
We tend to continue to tell this story today, because it's an easy story to tell...that the innovations of music is linear and if you follow these certain composers, then you'll understand how music developed. It's a horrifically inaccurate view to how music truly was in the 19th century, but because it's easy to tell and easy to understand, it's the version we stick with.
Surely, there are a lot of bad composers that aren't well known anymore too that were popular. But there were a lot of performances by composers that we don't hear about today. And while some of the music is lost, we have a ton of music by these forgotten composers, and many of it at the touch of our fingertips through the International Music Score Library Project (http://imslp.org). I highly suggest checking out that website and search by the time period. You'll see a swath of composers that you likely have never heard of (hell, I'm almost done my PhD and I haven't heard of most of them!)
Anyway, I hope this helps a bit. As far as I know (and this is where others might be able to help), there really isn't a single source that goes into why we prefer "genius" or "innovators" and how that shaped music history. It's a common thread that you'll see though in music history and music theory scholarship however.