....because of the waters of the flood.
Genesis 7:7
"Q: After an animal dies, does it become a fossil?
A: Not usually.
Its
remains are quickly consumed by other organisms, including 
worms and
bacteria.
Marine organisms are often consumed on the way down, and if
they reach the ocean floor, there are plentiful microbes to make use of
the organic remains.
Otherwise, wouldn’t whale carcasses pile up on the
ocean floor after millions of years? After so much time, why don’t we
see stacks of whales, fish, turtles and sharks hundreds of feet deep at
the bottom of the sea? Even shipwrecks have a finite existence.
The Titanic is being consumed in our own lifetimes by rust-eating bacteria.
Unless an organism is buried quickly, it will not become a fossil.
Even then, there are underground organisms that continue the recycling
process.
Worms and other burrowing animals perform “bioturbation,”
plowing up and scattering the layers, allowing microbes to consume the
original biological matter.
Unless the organic material becomes
permineralized (replaced by minerals), it should disappear in a short
time.
On top of that, geological processes such as water seepage,
earthquakes, volcanic eruptions and plate tectonics eventually take
their toll.
The more time, the more damage.
Q: Why would anything remain
after tens or hundreds of millions of years?" CEH