You are falling victim to frequency bias. Popular books are popular – and especially before mass printing technologies, really popular. A lot of people may have tried to write books doesn't mean they're writing books good enough to dedicate an actual person's time towards copying them down.

Also, Juvenal was a poet. He most likely knew other poets, or aspiring poets, or at least people who liked writing. Your average, generally functionally illiterate, individual at the time is not trying their hand at writing books.

I imagine it's easier to attribute a fragment of writing to a well known work, rather than a previously unknown one.