A short life-span might also explain why they go into stasis as they (seemed to at least) did LV426.
I was trying to make a theory work, but it doesn't. However, for the record ---
I first thought that maybe they had short lifespans to stop them being too successful and running out of resources (hosts) as gigantic armies of alien warriors would soon deplete any environment they were in (I think the very first comic says that they also had natural predators on their homeworld, but outside of that - unchecked - they were to of the food chain).
But that doesn't really add up? Cos once a host is dead its dead. Still, maybe if the warriors keep dying out and can't hunt in vast numbers, it gives the hosts a chance to repopulate?
I also thought the alien was sick and dying at the end of the first one (this thread reminded me of that) - or at least there was something not right with it. As Xenomorphine indicates, they probably had no real reason other than "we need a reason for Ripley to survive" and threw it in there, but its great to extrapolate on!