Here\'s a hypothetical situation: Three organisms show up on early Earth one day--an immortal (non-aging) organism, a mortal, non-reproducing organism and a mortal reproducing organism.
They all go for a \"walk\" along treacherous shores. All three get killed by a landslide. Which one do we see today? The last.
Why are there no reproducing, immortal organisms? Well, bacteria exhibit this. Now consider this. If your species is going to be immortal, it must have happened very early in your evolution. So you are something pretty simple, not a walking, talking person, for instance. Now you have mortal creatures all around you. They, because of their rapid evolutionary pace compared to you, discover that you are food before you discover that they are food. You can\'t keep up with their adaptations, so you end up being stuck at the bottom of the food chain while everyone else gets, say, a nervous system.
I\'m not stating this as fact, only possible explanation. I think it\'s possible for immortality to happen in simpler organisms, but the mortal ones will prey on you. They get to change as a species more often.