It's something to do with a war that happened, it was in all the papers.
Britain, USA and Russia were there to enforce martial rule at the time. With the fall of the wall there was no further need which is why Russia left around 1994.
Supplement from 06/09/2006 01:51am:
It is probably something that will become less and less as the original reasons for being there diminish. The cost of supporting an army in modern times is prohibitive so, for financial reasons, it will happen when the political climate is more certain. It is always beneficial to retain a springboard from inside Europe to launch counterstrikes.