http://www.sarna.net/wiki/Unseen
Short version:
For various reasons, FASA (back in the day) "borrowed" mech designs from various Anime/Manga sources under "ambiguously legal" licensing agreements. The designs made up about half of the selection of mechs at that time and were fairly "conservative" by today's Anime standards; the Battletech fanbase LOVED them. Around the time MW2: Mercs was released, there were legal rumblings between FASA and Harmony Gold on who really held the licensing rights for the US market (maybe all outside Japan, not sure on this). FASA lost and those mechs became the "Unseen". Since then, there have been attempts to redesign them to appease the fans (known as the "Reseen"), but companies involved have found it too much of a headache (also, apparently many fans hated *some* of the redesigns).
Personally, I don't see why they didn't put it to a vote and let the fans pick their favorite designs long ago. It is only the visual design that is the problem. An issue I'm still not sure why it hasn't been solved. My biggest irritation with them missing is NOT because of nostalgia, but some of their weapon configurations are REALLY good. There aren't any other 3025 era mechs that have the same fighting characteristics of the Shadowhawk, Thunderbolt, Archer, Warhammer, and Marauder. Having them missing from the MW titles has been rather annoying for me.
Edited by MagnusEffect, 13 November 2011 - 08:15 PM.