The issue is not so much background radiation (you'd have similar issues with RF), and your SNR is going to be reduced because of diffraction (as you rightfully point out goes at r^2). However the SNR would still be much better for optics because diffraction scales as 1/lambda.

The reason why JWST did get an optical link is that people developing these things are rightfully conservative and optical links in space are really still under heavy development.