Two very different games. As mentioned before production costs. For example I don't remember there being much voice acting in Dark souls.
Also the fact that Tomb Raider is a huge franchise, movies so on. Its expected to do well because of its history, you would expect a half decent game with a long history to do well. Where as Darks souls was the spiritual successor of some other game I cant even remember at the moment. On top of that its dark Souls 1, so in some peoples minds a new game.
Darks would most likely have had a very small budget, relatively, and was more a "lets test the concept and see if anyone is interested", in the hope of creating a new franchise (seems to have worked out). Tomb Raider is a big franchise in need of a reboot of some sorts, their expectations would have been much higher than darks souls.
Im rambling but one last thing, consider how hard dark souls is, I think it would have been incredibly difficult to get someone to put the money up for a game were most people wont be able to finish it. I think it looks like it was a pretty risky move that hopefully will pay off for them. I new genre of sadistic gaming.
