Why did Hestia give up her ngôi vua, ngai vàng for Dionysus?

she's probably a very kindly, wise goddess. With her being the oldest of the gods and him being the youngest, I believe it is symbolic of the old giving into the new (a common theme in Mythology)

She thought is was time to have someone else on the throne

Hestia longed for a loving family. When war became a possibility in her family, she did all she could to stop it. I was not a perfect solution though because now there is an uneven counsel, five girls and seven boys.