The key thing about preservation is lack of oxygen. Oxygen is a very active chemical and it will break down organic materials. So when the large mammals and other Pleistocene animals fell into the tar pits and sank, they were located in an environment that lacked oxygen. Hence, they were preserved.

At the La Brea Tar Pits, crude oil has been seeping up to the surface of the earth along cracks and fissures through the rock forming ponds of asphalt in low-lying areas.

Click in LaBrea-1 to enjoy a picture
Again click in LaBrea-2 .

These asphalt ponds at the surface made the area around the La Brea pits a hazardous place for animals to roam, especially in the hot summer months when the semi-solid asphalt turns softer and stickier. Sometimes, leaves, dust, water etc. would cover the asphalt obscuring it from view and creating a trap that animals could get stuck in! Once one animal became stuck in the tar, other carnivores or scavengers would try to attack the stranded animal, sometimes becoming stuck in the asphalt themselves as well!

Click in LaBrea-3 for a nice picture of an animal in the trap.

So, not only does the asphalt create a dangerous place for animals where they may die, but because the remains of dead animals sink relatively quickly down into the asphalt where they are completely buried, their bones are very likely to be preserved as fossils. After the soft parts of the animal (muscles, skin, etc.) decay and are eaten away by bacteria that live in the pond, the hard porous bones become saturated with asphalt and sink into the pit, preserving them from further decay and breakdown that would occur if they were exposed to the elements at the surface.

Click LaBrea-4 to see the bones preserved

Great question! I hope this is a satisfactory answer!

In order to be able to find the remains of animals an animal has to die, then the bones have to be buried and then dug out. Very often, animals will die in some place and then not be immediately buried. Bones may get broken, washed away, and eventually buried far away. There are many rocks on the Earth that contain fossils, some more than others. The tar pits are so special because 1) they were a place where many animals died and 2) the entire animals were buried so the complete skeletons can be found.

The tar pits in the past were often covered with water or leaves so that the animals could not see the tar. As they moved over them, they would get trapped and sink into the Tar. This happened very often and also caused the entire animal to be preserved in the pit. The tar cannot destroy the bones so they will just stay there until someone finds them and there is a lot to find because so many animals died there.

Animals fall in, die, and nothing then can eat them because anything going in also gets stuck in the tar. The bones are entombed in the tar and preserved.

Good question! Bones are preserved in La Brea Tar Pits because sometime long ago, the area was covered with sticky tar (much like oil but stickier) that trapped the animals that used to roam around there. The tar is the same thing that we use when we build roads. Because tar is very thick and sticky, the bones are protected from natural climate (wind, rain, etc.) that would have eroded them otherwise. No oxygen can get through, so the bones don't rot or decompose.