Ray I think you nailed it with demand. There is not enough need or demand for manufacturers to install the kind of soundcard needed for serious music production....so they put in standard cards that are jack of all.
Remember it is not just the quality of sound and latency that separates the top end soundcards but connectivity and their interface capabilities.... you cannot connect multi track and many input devices etc to a standard card. No point in putting in bells and whistles and connectors that very few would ever need and use.......PCs do not have the space required nor would the average user want to fork out the extra expense. That said if we did not have the specialist cards and interfaces with all the specialist gizmos then simultaneous multi track music production to studio quality would be all but impossible.

http://www.soundclick.com/bands/songInfo.cfm?bandID=687727&songID=5238055