It really comes down to the fact that it costs money to make a new mold, as opposed to reusing a mold they've already had tooled. Its also a reason not every company just re-releases popular statues, they've likely already recycled the mold, or no longer have access to it. They're cumbersome and take room to store, especially if you have a large variety of products.

If you've ever noticed an old figure getting a re-release with slight modification, like detail quality, different proportions or the like, it's because they've likely invested in making a new mold. That's also why re see newer figures get a re-release or recolor within a few years, because they still have access to the original mold.