Why does the ankylosaurus have the nickname armored tank?

They probably called Ankylosaurus the "armored tank" because it was so densely covered in bony armor plates on its head, neck, back, sides, and tail, just like a tank is armored to protect it from gunshots. Some other things Ankylosaurus shares with tanks are that it was bulky and hard to flip, and they were both well armed: a tank has guns, and an Ankylosaurus has a lethal tail club.