"Dinosaurs" are just animals like any other, they are just extinct. It's just a term they tagged onto them, one could call a dolphin a dinosaur in that respect. Sometimes "science" make no sense. They were most likely loaded into Noah's ark just like all the other animals, there certainly was plenty of room for them (babies). They were generally just too big to live on the transformed Earth. Before the flood there was a canopy of mist that covered the earth and made plants grow in abundance that these big animals could eat, but after the flood it was clear skies which could not support food so well year round, so they died in regards to the largest ones. For the rest, just like any animal, there is a danger of extinction due to the sin of Adam which had consequences for all creation. God didn't create death, that is a result of sin, the sin that Adam commited, so animals die since then as well as does man.