Actually there are hundreds, perhaps even thousands of explanations. Many disagree with each other and virtually all are contradicted by science. But there is no shortage of explanations!
And now for something totally different:
Q. What's harder than getting a pregnant Brontosaurus into the ark?
A. Getting a Brontosaurus pregnant in the ark!
(Noah! Make them stop. I'm getting seasick!)
LOL.
How big was the barf bucket? (see tagline)