TheDudeLebowski: Guessing it's a safer business venture than a completely new IP. At least, that's how it works with movies, so I'm assuming it's similar with games.
Being reclaimed by the publishers/developers and re-introduced to a newer audience may also be another reason (LucasArts' stellar adventure games come to mind).
Or when a game has long sense been dead for example Strife it was gone from everything and was pretty much abandonware only for awhile till people got ahold of it and rereleased it with added factors.. thats a case that makes sense..
I also can get Sin being remade because the O.G release and version not only is good it failed for Half life over shadowed it and it was a pc game in the early days of''release unfinished game patch it later '' bull and do to the early days of that it failed hard as it not only was from my findings a bug ridden mess fior a month after release but the patch one had to search up and go to a forum or dev/studio page to get was massive for just bug fixs and one extra map atleast for the time it was massive.. So ya remaking it now might give it a shot to thrive which I want to it thus hoping the ''Sin;reloaded'' game thats coming out soon which is a remake I am very hopeful for and do hope it comes to G.O.G and its one of the few games I will buy on Steam and G.O.G rather then just G.O.G via waiting for it to show up as its a game I want to not get screwed over again