I'd wager this is because there is one outright leader in the field (the Contributor Covenant) who's author has at times been a controversial figure in her views on enforcement and scope of codes of conduct, which can lead to people being apprehensive of just adopting the popular example.

You can see similar with licenses. Stallman was initially controversial for his views on software licensing, and so in the early days there was a proliferation of licenses like the eclipse public license, mozilla public license, microsoft public license, CCDL, etc.