The beavers are still there, just not in high numbers like they used to be. Right now the ecosystem can only support the population that is already there. Maybe eventually it will support more and then the number will increase naturally.

TFA implies the issue is land owners won’t allow it:

> But beavers can only thrive if landowners allow them to.

> Maybe I missed it, why didn't they just reintroduce beavers?

My guess is that trees are not there in numbers enough to support beavers. Or maybe they don't want beavers chopping the trees.