Ethereum and other blockchain-based platforms have lured thousands of volunteer developers with the promise of alternatives to Google’s Android or Apple’s iOS that can’t be censored by governments or co-opted by corporations. What they don’t have yet are users.

The creation of decentralized apps, or dapps, has been one of the most hotly anticipated developments of blockchain, the technology that underpins bitcoin and other cryptocurrencies. Investors have poured billions of dollars into the effort, yet they remain largely...