There are so many prizes for first novels that it gets our contrarian juices flowing. There's a kind of crossover syndrome from the world of rock music, because usually, with a rock band, their first album is their best. The opposite is the case with most writers, especially the very good ones. It often takes several books for them to really hit their stride. Writing is something that takes a long time to learn to do.
Every writer says so.
This focus on the first novel and getting the book out while they're still young and thin puts the emphasis on what tends to be the weakest work of someone's career at a point when they're really trying to make a reputation and needed to ramp up more slowly. One case that comes to mind is Michael Cunningham. His first book isn't even listed on his subsequent novels. I've been told in no uncertain terms not to even bring it up with him because he's so ashamed of it. Another thing that happens is that younger writers think that since they've made it they don't even need to work on their craft.