A factor in this case may be the fact that while desktops remain a very sizeable segment in the PC space in the Mac space an actual traditional desktop is available only as a very high priced workstation which starts at $7000 whereas more traditional users are apt to be served by minis or imacs.

There were probably about 1000x more Linux desktop users than Mac Pro users willing to install their own aftermarket GPU even when this was a thing.

There is another group of macOS users who were buying Nvidia cards: hackintoshers (those who hack macOS to run on generic PCs). For several years, by far the most cost effective, powerful, and flexible “mac” available was one that you built yourself from a selection of known-compatible components. Many in this group were buying Nvidia GPUs for use in these machines since Nvidia was furnishing macOS drivers at that point.

It’s hard to tell how numerous this group was, but I saw them come up surprisingly often in photos and discussions online, even appearing in production environments (despite legal dodginess). Wouldn’t be surprised if at times this is where most Nvidia cards purchased for use with macOS were sold.