1. I want organizations (initially newsrooms that care about data journalism, but rapidly growing beyond that) to be able to trust the product. The default result of 95% of startups is to go out of business, and going all-in on a product that then blinks out of existence is bad! When I'm selecting a vendor this is a thing I always consider, so I'd like my customers to feel confident that they have the ultimate escape hatch - run it yourself - should they ever need it.
2. Newsrooms in particular have learned this lesson before, many times over - and not just for startups. Remember Google Fusion Tables? https://en.wikipedia.org/wiki/Google_Fusion_Tables That's just one relatively recent example.
3. Datasette is an ecosystem play. There are over 100 plugins already - https://datasette.io/plugins - and I hope for there to be thousands more. Developers don't invest nearly as heavily in building and releasing plugins for closed-source platforms.
4. Open source is a way for me to punch _way_ above my weight. I can have a much larger impact on the world by participating in open source, compared to if everything I'd been building had been closed.