Because the awarding of the Medal of Honor is done by an act of Congress. The US Medal of Honor was authorized by the Congress in 1862. The first Army soldier to receive the Medal was Private Jacob Parrott.
If you meant the series as a whole, there are many other Medal of Honor games such as Allied Assault or Pacific Assault. If you meant the newest Medal of Honor game, there is no sequel as of October 2011, but there is rumor that a new Medal of Honor game is in production.
There are many websites (some official) that list all Medal of Honor recipients. Do a websearch by simply typing in Medal of Honor. I suggest this only because there were 96 recipients from WWI - TOO MANY to list.