Why was my B&W image rejected?

I think, that besides being black and white, zou also have other qualitz issues with this picture. You should really check the guidelines. If you are new to stock, you should consider these resources: https://helpx.adobe.com/stock/contributor/tutorials.html
Please read the contributor user manual for more information on Adobe stock contributions: https://helpx.adobe.com/stock/contributor/user-guide.html
See here for rejection reasons: https://helpx.adobe.com/stock/contributor/help/reasons-for-content-rejection.html
and especially quality and technical issues: https://helpx.adobe.com/stock/contributor/help/quality-and-technical-issues.html

Have you read the contributor user guide yet? Had you done that you would already know that monochrome conversions aren't accepted.

While I have had B&W and monotone images accepted (AI generated) they were still submitted as RGB images. Images that are taken with a camera and then converted to B&W are rarely if ever accepted. But there are other issues as well. First, I'm not seeing what value this image might have from a commercial standpoint. The contrast seems extremely high, and there seems to be some over-sharpening and loss of detail as a result.