Because horror movies are so reliant on the quality of the special effects and the authenticity of the makeup, up-to-date remakes usually have a distinct advantage in that they have access to a more advanced pool of resources than the previous generation of filmmakers did.
Horror flicks are also easily capable of generating high returns off small budgets - anyone with access to a spooky house, a creepy mask and something stabby could whip up a half-decent flick in a matter of weeks - so it's no surprise that studios frequently revisit cult classics or fan-favourites and give them a modern lick.
The genre has seen an insane amount of remakes over the years - probably more so than any other type of film. It's not all good (hello Rob Zombie's Halloween, 2013's Carrie and the recent Flatliners), but hidden in all the rubbish are plenty of gems that are worth your time.
The word "remake" usually has quite a stigma attached to it, but ultimately, they can only be a good thing. If they're bad, they'll at least draw attention to the superior older version, and if they're good - like in the following cases - we'll have a more refined version of the original movie.