I found a site that has the best description of the 590 I've ever seen. Here it is..
http://www.tacticalshotgun.ca/content_nonsub/shotguns/compare_870_590.html
http://www.tacticalshotgun.ca/content_nonsub/shotguns/compare_870_590.html
My guess would be the fact they already had them in inventory. If you know Pat Rogers, you know what he claims the Navy-Marines think of the Mossy.Speculating as to why remington did not want their 870 to take part in the trials,