If by “good” you mean in terms of being able to play a wider variety of music than existed in the late 18th century, I’d guess it’s because they have 200 years of experience playing the music that came out later, specifically after Beethoven began to write.
Many musicians of the day were simply unable to play Ludwig Van’s later works because they’d never seen anything like them before. Now we’ve grown accustomed to hearing them, and works like them that later composers following Beethoven’s lead produced. Like much else in that period, Beethoven was revolutionary!
It is truly amazing to consider how much music has changed since the days of Mozart, Haydn, Haendel, Bach. Much of the difference is also due to technology and the new instruments it provided, e.g., valved horns and hammer pianos. This allowed new forms of music to be created and flourish alongside of (and sometimes displacing) earlier styles and genres.
Listen to the works of the earlier composers and then compare them to what came later. The difference is as clear as night and day!