Population decline + expanding landmass.
So not only do we lose people, the universe gets bigger, thinning out what people we still have.
Play interaction is key in any MMO, but even more so in games like Siege. The "hardness" factor...the risk....the danger, all declines with population and expanding land mass. Which is why I'll never understand people who like Siege for the "hard, risky, dangerous" shard they think it is, yet oppose changes, such as changes to ROT, that will increase its population. Or oppose changes that will condense its population, such as removing land and preventing the addition of more.