Printing can be on a substrate that is adhered to a lens or directly on the lens itself. When printing directly on the lens, the ink is applied to the flat side of the lenses. In this printing method it is important that the printer is capable of aligning the interlaced image directly with the lenses. This level of precision limits the number of printers capable, and to some extent the speed, of the print. If a sheet is fed through a printer at even the slightest angle the interlaced printed lines will not match the lenses and the result will be a jumbled mess with lots of ghosting and blurry images.
This video shows a high end printing putting down an image directly on a sheet.
For more about lenticular see 3dEngr's 10 things lenticular.