Securities law. See the JOBS Act.
Anyone who invests more than $10k towards a company counts as a shareholder towards the 500-shareholder limit for a private company. If the company exceeds that limit, they are treated as a public company when it comes to SEC filings. From this, you can infer that SEC filings are a pain, especially for small companies.
*Clarification: Kickstarter's original donation model was not subject to SEC requirements, but the JOBS Act makes the $10k-in-exchange-for-equity business model viable.
*Further update: Kickstarter's official position on offering equity: "We’re not gearing up for the equity wave if it comes. The real disruption is doing it without equity. The real disruption is when you break down the funding of a project into all these little bits." - gigaom.com/2012/05/22/kickstarter-founder-perry-chen-intervie.
One of the principle drivers behind the IPO filings of Facebook and Zynga was the 500-shareholder rule, a vestige of the Securities and Exchange Act of 1934, which said that any company with more than 500 shareholders has to open its financials to the SEC like a public company. But under the JOBS Act, anyone who gives $10,000 or less will not count toward this limit. The act also raises the shareholder limit from 500 to 1,000.