I think there are a lot of reasons. I like all sub sims, but the German subs of WW2 are a particular interest, as are Cold War Russian subs, so I think that might mean that, like in movies, I find the 'bad guys' more interesting (that's not meant to be an insult to Russians by the way, or Germans for that matter, I just mean in terms of having grown up in the post WW2 Cold War and having that point of view rammed down my throat for years).
Also like in the movies, the bad guys tend to have all the cool looking stuff, and ironically enough, that seems to be the case with both the Germans and the Russians. I think the German WW2 subs just look a lot better and had some more interesting weaponry and defensive conceptual stuff such as radar absorbent covering material and programmable torpedoes etc.
That goes for the land forces and air forces too - a Spitfire is a pretty aeroplane, but a Ju-87 Stuka looks like it would kick your dog to death and laugh afterwards, it
looks like a warplane with its angular shape and jagged camouflage. The Tiger and Panther tanks had all that anti-magnetic zimmerit paste on them and interesting paint schemes in comparison to allied stuff with a quick spray of olive drab. That's the case with Russian subs too, I mean we all know the LA Class sub is great, but the Akula and Alpha look a lot more groovy and mean.
Compare it to a sci-fi concept if you like, the X-Wing is okay, but the TIE fighter looks waaaay cooler and has that mean howl, not unlike a Stuka's siren (incidentally fact fans, the TIE fighter noise is in fact an elephant 's call slowed down and messed about with).
I like the bad guy's stuff.
Chock