Because there is no problem of overpopulation, only a problem with a world economy that does not provide people with what they need. Scarcity is not the reason people don't have their basic needs met; capitalist hoarding is. From my profile:
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Global spending in 1998 ($U.S. Billions):
Cosmetics in the United States 8
Pet foods in Europe and the United States 17
Business entertainment in Japan 35
Alcoholic drinks in Europe 105
Military spending in the world 780
Estimated additional costs to achieve universal access to the following basic services in all developing countries ($U.S. Billions):
Basic education for all 6
Water and sanitation for all 9
Reproductive health for all women 12
Basic health and nutrition 13
http://www.globalissues.org/article/...acts-and-stats
"The United Nations Development Program (UNDP) reported in 1998 that the world's 225 richest people now have a combined wealth of $1 trillion. That's equal to the combined annual income of the world's 2.5 billion poorest people."
http://www.globalpolicy.org/socecon/inequal/gates99.htm
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And if by overpopulation you mean
eventual overpopulation, then I guess the reason it's not a left issue is because the answer is so obvious that it's not really divisive: family planning. Any one with half a brain realizes this, whether they are on the right or left--a few "abstinence only" fundamentalist Christian morons in the Bush administration notwithstanding.
This book does a good job of tracing two different historical schools of thought on this issue: the Malthusian view, which denies a right to subsistence, and therefore focuses on (over)population; and socialism, which firmly defends a right to subsistence, and therefore focuses on wealth (mal)distribution. So if anything, I'd say that overpopulation is a right-wing obsession, not a left-wing concern.