Your project being popular is enough. I had no idea who Andrew Kelly was but after reading Zig's documentation, trying it, and feeling like it satisfied what I want from a language of that style going forward, I started chipping in a bit each month.

It probably helped that I watched his video demos on YouTube and such and saw there was a person behind the project.

I'd imagine the project being something exciting or something that makes programming more fun helps a lot as well. An extremely useful but boring project probably won't inspire as many people to donate, even if that's not really fair.