For three reasons.

First, you can reduce the population, but all that does is slow the rate at which non-renewables are used up. Then you're back to the Amish scenario.

Second, all our eggs are in one basket. All humanity on one planet increases the risk of extinction. Colonize Mars, create off-world populations, and you massively reduce the risk of one shot killing everyone. No T-Rex Mars colony meant no more T-Rex.

Third. Given the nature of mankind, population control wouldn't necessarily be a pleasant thing. Think of the means by which such a mandate would be attained. Think eugenics, holocaust or, if you like Conspiracies (and I do) the real meaning behind Agenda 21.

We need to to industrialize the exploitation of space, and then we need to start getting sustainable colonies up there.