Because remakes are big money. The series was essentially dead for many years but I assume they still want to do something with it at least one more time. To ensure a profitable game plus ignite the franchise again, they are doing a remake....if it does well...then I would assume we would see a new Dead Space game in the future.
Ubisoft is doing the same thing with Splinter Cell...though remaking the first game could be a pretty cool thing considering how old it is (and doesn't age that well)...