Raymond outlines his reasons for refusing donations on the ublock wiki, if anyone is curious: https://github.com/gorhill/uBlock/wiki/Why-don't-you-accept-...

One thing he (rightfully) points out is that the real power of ad-blocking solutions comes from the community-maintained blocking lists. In theory one could manually block every url they run across, but in practice we can use these tools (or a tool like pi-hole) almost effortlessly.

It's free (libre) software; you don't owe anyone. Eventually you will encounter a bug or translation/documentation error in some free software; taking some time to fix that would help everyone. And so would making software to fix a problem and then releasing it under a free license.

It might not happen in the near future, but chances are eventually you'll get to contribute something. And even if you don't, just accept uBO as a gift from a nice person.

He wants people to donate to the people compiling the block lists.