Oxygen. Specimens trapped in ice or tar or are quickly engulfed in a muddy river bank are sealed in an airtight tomb where things that would decay them can’t reach. Namely oxygen to fuel bugs. So, they sit.

Things that die in a warm/wet climate crawling with trillions of insects and scavengers won’t last a few hours.

We know so much about the past largely because bad things happened a lot. There were at least five mass extinction events that nearly killed everything. Those are easy to find if you dig in the right spot. Harder to identify and locate are the isolated events of mudslides and flash floods and so on. With that in mind, when digging, you want to locate the layer of dirt relative to your timeframe and root around in it. If you’re looking for a needle in a haystack, you’ll rarely find it. If you’re looking for a planet killing event, you’ll find that layer easily, AND the specimens trapped under it. So, paleontologists spend a lot of their very valuable time digging easy and well known places like the black hills, or riverbanks in Canada. Because of all that, we have lots of fossils from specific eras and almost nothing from others.