It's not the only size available.
Many "mass market" paperbacks have smaller 5 x 8 pages.
Some "gift" books and photo books are 8 x 8.
I own a book about sports cars that measures about 30 x 40. If I added legs, it could be a desk or table.
Some reference books, including a famous copy of the U.S. Constitution, are small enough to fit into a pocket.
If your book is only text or mostly text, the size doesn't make much difference—as long as the combination of page size, typeface, type size, and margins provides an attractive and readable book.
However, if you are going to include photographs, illustrations or charts, bigger is usually better.
I had a rude awakening when I bought The Step-By-Step Guide to Self-Publishing for Profit! by Christy Pinheiro and Nick Russell.
The book is aimed at writers who want to use Amazon's CreateSpace publishing service (now being absoirbed into KDP). The book is good-looking, well-written, accurate and useful.
But what most attracted me—and made me jealous—is its 7 x 10 page size. That size is an inch bigger than my books in each direction, and provides 16 more square inches of page size.
The big pages have room for lots of "air" (white space) which makes the pages very appealing and accessible, and provide lots of room for computer screen shots and charts.
The popular "Complete Idiot's" and "For Dummies" books are about 7.2 by 9 inches.
If I was not such a complete idiot and a dummy, I might have used bigger pages in a couple of my books.
If you're still in the planning stage of a publishing project, consider going beyond 6 by 9. Keep in mind, however, that if you have bigger pages, you'll probably have fewer pages. When people read a book's description they are more likely to notice the page count than the page size, and a book with only 96 pages may seem skimpy.
Another thing to consider: page size may not affect what you pay for printing. A book with pages up to about 7.5 x 9.7 inches may not cost any more than a 5 x 8 book, and since you can get more on each bigger page, with fewer pages you'll pay less to print each book. You can charge less, or make more, or both.
On the other hand, a book with bigger pages won't fit into the convenient and free 9.25 x 6.25 x 2 Priority Mail "large video box" that can be mailed for as little as $5.05.
Size isn't everything, but it's important. Think about it carefully.