To quote from the Epson site...
Wedding, portrait and school photographers have traditionally used luster paper for their photos. Now Epson offers this popular surface paper, Premium Luster Photo Paper, to digital printmakers.
They forgot to include digital photofinishing.
And that's the land of 4x6, 8x10, 16x20 and other standard sizes conveniently cut from a 16" wide dimension.
We artsy photographers are just tiny blips on the map in the much larger world of what 98.6% of inkjet media is actually used for! An extra inch of width would look like money-sucking waste to most of those high volume users.