It's mostly due to functionality concerns. Making something thin then dropping to a 5400 rpm hard drive is dumb, especially when users can't easily swap in an ssd. I'm referring to the 21.5" version. It's the cheaper one, and that is a major sticking point for such a machine. Thickness is something you should never perceive in a desktop display anyway. You look forward at the display when it's in use, and it's still using the same stand. It's not like you're gaining any kind of space. It doesn't open up new potential for placement within a work area or home. Then of course there are concerns over thermal levels. This is an area where Apple has made compromises in the past in the interest of thinner machines. That is something that has no place in stationary form factors. The density should really be a secondary concern after all other concerns are met. It's not like a notebook where you'd take it with you. Very few people would transport an imac.