It would be much easier to evolve better reproduction skilll than better longevity. To have more offspring, alll a species may need could be longer claws, stronger legs, stripier skin, etc. To live longer, the organism would have to change its cells on a more fundamental level.
In the wear and tear of daily life, hundreds (probably more, but I don’t want to be wrong in my guess) of cells die. They are sequentially replaced by new ones. (Yes, even nerve cells, although on a much slower timescale). Cells in most multicellular organisms have special tags on the ends of the DNA (I think they’re called telomeres, I may be wrong). The designed purpose of these tags is to protect the ends of the DNA (Appaently dividing is strenuous on the integrity of the chromosome). So each time the cell divides, rather than lose a chunk of the important information, a piece of the telomere is lost. Since the telomeres have a finite size, eventually it’s all going to be gone. At that point, the cell can no longer safely divide and so it stops. Once the cell stops dividing, damage can’t be repaierd. Decay really starts to set in, and the animal dies.
On a side note, research is currently being done to stop telomeres from breakign off. They’ve been able to keep a piece of foreskin dividing for two years in a culture, much longer than it naturally should have. Unfrtunately, cells that divide infinitely increase the risks of cancer, so there are still a few kinks to work out.