Who decides to give a reissued book a new cover and title?
That is strictly up to the publisher. The author has no say on when or how a book will be reissued. However, when a book is reissued years after the original publication date, it only makes sense to give it a fresh look and title that reflect the current times. It is not meant as a way to dupe readers into buying the same book, but rather it’s a means to let new readers acquire some hard to find copies they may have missed earlier.