because it is a nice, damp and warm medium (which quite possibly gets pooped on) bacteria and mold can grow in it.
I replace mine yearly.
You could "wash" it with boiling water or by baking it, and thus use it longer, but its not worth it (sand is cheap).
I do this as well, I've got the dirt I use in mine out in the sun as we speak, getting "freshened up," as it were. It was starting to get little gnats, so I put it outside to dry up and clear out anything that's living in it before I re-moisten it and put it back in my females' cages.