Why Are Most Crime Novels Bad?

Because they are part of a series. And books in a series eventually run of steam. The author runs out of ideas and begins recycling old plots and old concepts and he or she doesn't really care because they know the books will sell. Publishers and bookshops love series because people buy them without thinking. And then read them without thinking. It's very rare that series titles retain quality after say book 5 or 6. They've almost certainly lost credibility by that stage because no character could possibly go through that much and not have a nervous breakdown (although clever authors include the nervous breakdown as part of the plot.)
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In an ideal world only first novels would be published. The new writer has something to say and they put everything into that one book - all their jokes, all their sylistic quirks, all the things that make them weird and interesting; the book would come out and it would either be a hit or a miss and then someone else would come along with their story. Wouldn't that be great? Readers would be continually getting new and original voices and because no one would have a reputation publishers would have to rely on a writer's talent alone. Alas that isn't how the world and especially the crime fiction world works. The books that sell a lot of copies are by established professional writers churning out series titles. Readers are reluctant to try new things and the non crime fiction reader who browses a random one of these books is put off because the novels are generally beyond terrible. When I was down at the caravan site in Warrnambool last week I talked to a bloke with the new Tom Clancy novel. "How is it?" I asked him. "It's awful, but what else am I going to read?" he said cheerfully.
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The only series I know that maintained quality throughout (except for the penultimate novel) are the Patrick O'Brian Aubrey-Maturin books. If you know of others I'd be happy to hear about them...