No place else to go. Hardin thinks this is not a good metaphor - implies that everyone has a right to all of the resources equally. Ignores the essential fact that a spaceship has a captian to allocate resources - earth does not.
Poor nations are crying out for help - they want to come aboard our boat.
There are 100 people in the water crying for our help. Take them all in. If we do this, our boat sinks and it is likely that we will all drown. While we might wish we could help them all, reality is that we cannot. Take in 10 - up to the capacity of the lifeboat.