One explanation I heard was that after the third creative day, there was all this vegetation run amuck, since there were no animals to eat plants, so Jehovah made these big critters to mow it all down. Then of course, they had accomplished their purpose and they died off. Not sure this is actually out of a WT publication, but I did hear it at a Kingdom Hall.
The Aid book, printed in the early 70's, declares that if any dinosaurs were still alive at the time of the flood, they would have been wiped out at that time. Noah of course took two representatives of the lizard kind into the ark, just not dinosaurs.
Yes, this was the 1970's.
I am not making this stuff up.