Dinosaurs were on the ark like every other creature. Dinosaurs hatch out of eggs, eggs are not that large. Even the dinosaurs that did grow to huge sizes, of which there were few of, started out as small animals hatched out of an egg.
Once all the animals came off the ark they faced a new hostile world. Do you know how many animals will go extinct just this year? if you look up a secular resourse on this you find:
"Experts calculate that between 0.01 and 0.1% of all
species will
become extinct each year. If the low estimate of the number of
species out there is true - i.e. that there are around 2 million different
species on our planet** - then that means between 200 and 2,000 extinctions occur every
year."
So lets say 200 animals will become extinct just this year -where are they? They are gone.
The dinosaurs became extinct just like all these other poor animals.
How? How do other animals become extinct?
They get diseases, they have food shortages, they face catastrophic events like fires and floods, man hunts them.
Right from the get go off the ark all the animals were in great peril of extinction. There was only 1 pair of non clean animals and 7 pairs of clean animals. Dinosaurs were among the non clean. To add to that we have hungry humans who God told could now eat meat.
Genesis 9:3
Everything that lives and moves about will be food for you. Just as I gave you the green plants, I now give you everything.
God just granted them permission to eat them. Animals that hatch from eggs on the ground would be among the very first things that mankind would eat. You imagine yourself in the wilderness. Is it easier to catch and kill a moving creature or to raid a nest while its not attended? No doubt they did both but eggs are an easy target.
It's not a wonder that dinosaurs are extinct, but rather a wonder that we have as many animals non extinct as we do.