Because the original blocker: AdBlock, did. And some donated more than others. The rest is history.
That's unrelated to donations. One can accept donations without putting their soul on sale.
As an extension developer I get offers for buying my extensions or adding donator-specific functionality: I just don't accept them, even though I accept simple donations/sponsorships. The extension is still free of ads (except in the readme)
https://blogs.cornell.edu/info2040/2016/09/15/justin_kuang_a...