Well, the short answer is, "They don't get trapped in sediments that take years to accumulate." Compared to the astronomical number of individual life forms throughout history, fossilization is a pretty rare event.
Generally you find fossils in situations where bones aren't likely to be disturbed. For instance, tar pits. But some predators , who tend to drag carcasses back to safe places to eat, tend to dicard bones into piles. Sometimes these garbage piles go undisturbed until they've covered in mud or something.
The question isn't "can fossils be created by catastrophic means" because the answer is clearly "yes they can". The question is, "Are catastrophic means sufficient to explain the deep, broad, and generally well-sorted fossil record?" The majority of geologists and paleontologists say the answer is "no."
(p.s. I'm no paleontologist so the examples of fossilization I've given shouldn't be taken to be definitive.)
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