16:9 are physically smaller and use less material yet able to maintain the same screen size specification as a 16:10 monitor. Hence such difference is often missed/ignored by the ordinary customer and hence manufacturers are able to get away with this. In other words, 16:9 monitors == higher profit margins
The way I understand it, these panels are all basically cut from one continuous sheet in the factory, so it's difficult to re-tool a line to produce "taller" panels.
