More here. The short story: nonvoters have different attitudes, on average, compared to voters. When lots of people don’t vote, this affects political discourse:
An example is health policy. As my colleagues and I have found in our analysis of national survey data, attitudes on health care reform vary mostly not by where you live but by who you are. Older and richer Americans solidly opposed universal coverage for the uninsured, while younger adults, middle-income and below — who turn out to vote at much lower levels — were much more supportive of such a policy.
Well-educated rich middle-aged whites (e.g., myself) vote at something like a 90% rate in presidential elections. I’m guessing that if you consider the fraction of the population that has lots of money in mortgages or the stock market, most of these people vote, and many of them contribute financially to election campaigns.
And, unsurprisingly, when housing prices fell and the stock market was crashing in 2008, that was considered by the government and the media to be a crisis that needed immediate attention. Both parties felt the need to respond right away, as votes obviously hung in the balance.
On the other hand, economic concerns that hit nonvoters and noncontributors get slower responses. When you get big gaps in voter turnout, you get big gaps in political representation — and millions of voices go unheard.