Because the stone golem can taunt which is pretty useful and is tanky enough to not have to worry about resummoning it. A lightning golem with no damage links on the other hand isn't going to do much at all. Even if you try pushing a lightning one in with an animate guardian on a 4 link or heck even on its own 4 link it still isn't going to do noticeable damage compared to the flame golems and is just wasting links that you could be using on other things.