dick1982: ^ RedFaction??? Why ??? Because communist symbolism?!?
Because it's a first person shooter that has blood and violence in it. For quite a while, the BPJM (or BPJS, as it was called before 2003) basically yanked everything that seemed too violent (officially "harmful to minors") to them. In the 80s, this included games like 1942 or Commando. They were quite rabid about putting games (and movies) on the index.
That's why Star Wars: Dark Forces and The Chaos Engine are to be locked as well. Even German censors would probably consider them laughably tame nowadays, but an entry on the index always lasts for 25 years, unless it's renewed (which would add another 25 years) or the publishers of the games/movies/books/etc. in question get the indexing repealed prematurely.
However, thanks to a change in laws in 2003 and a generally more relaxed attitude towards these things, the situation has eased up quite a bit. Today, even games like Shadow Warrior 2013 or Doom 2016 are officially available to us Germans. These old games are mostly just "sitting out their sentence", as it were.