Thats because to be fossilised, an animal has to be laid down by water and buried quickly.
Exactly, a global flood would provide ideal conditions for fossilisation. Rapid deposition of sediment cuts off the supply of oxygen, slowing decay processes, and any scavengers which may scatter the remains would also perish. In fact for such a successful method of fossilisation, it's a wonder there are not more fossils found.
Consider how abundant fossils would be if this global flood hit the equivalent of the Serengeti, for example. Another example would be the ruins of Pompeii and Herculaneum. Although a result of the volcano Vesuvius, and not a flood, we still get an idea of what to expect from a sudden disaster, with hundreds of skeletons having been discovered.