I don't know, having more color doesn't seem that much more expensive? I guess the fabrics are expensive and stain relatively easily, but if public transport can keep them clean so can airlines IMO. Some fabric seats are over 3 decades old and they're still in a really good shape.

Maybe it looks better because there are only 9 seats in an aisle in the picture compared to standard 10 nowadays?

Public transport can’t keep them clean. BART famously had disgusting carpets until the 2010s.