I heard that a DS1 remake would be a big undertaking because it I believe right now DS1 runs on directx9 and doesnt run as efficiently as some of the newer APIs. They would either have to try to improve the visuals/textures/framerates of the current version or do a complete new game with the same world, story, characters, etc. Either way would cost them money, and from what I've heard, it would cost them more to remake the game than they would receive in profit.
Again, this is just what I've heard. I would love to see a remake myself, but I'm not holding my breathe.
Please feel free to add to what I've said or correct me.