Both Canon and Nikon have made some great lenses for their mirrorless mounts, but popular manufacturer Sigma has yet to make anything for the R or Z mount. Sigma's CEO says that won't last forever, though.
A recent interview with Sigma CEO Kazuto Yamaki for ICAN discussed the company's plans regarding producing lenses for Nikon's Z mirrorless mount and Canon's R mirrorless mount:
It’s true that the photography market has been systematically shrinking for years — in keeping with more and more people simply using smartphones to make photos and videos. Less cameras sold also means less potential customers interested in buying a lens, which is why our short‑term strategy indeed involves a possibility of introducing lenses with new mounts — to such photography systems as Nikon Z or Canon EOS R.
Yamaki qualified the statement, however, mentioning that at the moment, the company is trying to catch up to demand for its current lineup of lenses and that that might delay its ability to develop newer lenses.
I think it is likely only a matter of time until we see Sigma lenses for Nikon and Canon mirrorless mounts, particularly since both companies are moving away from prioritizing their DSLR mounts. Certainly, third-party lens options will be welcomed by photographers.
Maybe by the time they make Z-mount lenses, the Z 6II will be in stock somewhere and I can finally get one...
Actually I read the interview via NikonRumors and they were saying that even if it is possible that they will produce something in the future, for now is a no because they haven't the R&D resources to produce lenses for new mounts and they can't keep up with the actual production. I'm Italian and my English skill doesn't shine, but it doesn't seems to me that they were likely to do anything soon...