Probably submarine simulations are harder to create than "first-person shooter" types...
A submarine is something very complexe (you can still visit existing WWII submarines to understand what I'm thinking about), and it operates in a complexe element (the sea)... Moreover, the battle of the Atlantic was also a very complexe battle which lasted for several years... So, not very "money-making" simulation in the end... and also because it's played by far less people...