The reason is that Braille can't really be resized and still be readable, so letter cells are fairly large. A normal letter / A4 sized paper fits maybe 28 columns of text, so Braille is often embossed on legal size paper. And Braille pages don't lay flat against each other, so the books end up being enormous. The paper itself is also thicker because it holds dots better, so the books are quite heavy. This is why so many contractions are used in printing Braille.

Some languages use few or no contractions in Braille, but I think many of them also have very few Braille books available.