The reasoning is specific and pretty interesting IMO. Contributions are accepted as long as they stay public domain.

“In order to keep SQLite in the public domain and ensure that the code does not become contaminated with proprietary or licensed content, the project does not accept patches from people who have not submitted an affidavit dedicating their contribution into the public domain.”

I’d guess this compares to the GPL in interesting ways too (i.e., unlike GPL public domain code can be used/copied/distributed without license, even in proprietary settings and for profit, but attribution, credit & copyright must be waived first).