Color E-ink screens are expensive, and that is doubly true as the dimensions increase. There’s less of a demand for large screens, which means smaller production runs, and consequently higher costs.
But those larger screens are available, if you have the funds.
I just learned that E-ink is now selling a 31.2″ Kaleido screen unit for $2,300. (You can also buy a grayscale screen for $1,500.) This is just the screen without a controller chip, power regulator, or anything, so it’s really more for the DIYer than consumers, but it is available.