It's because of macOS Catalina, which no longer supports 32-bit software. Becuase Cities in Motion was created as a hybrid of 32- and 64-bit code, the developer had found that the code would no longer run on all-64-bit Catalina without serious revisions, and deemed that it wasn't worth revising the game for the subset of people (like us) who are using Catalina. Also, GOG, to protect Catalina users, chose to delist it forthe vast majority of Mac users.
Blame Apple and their decision to cut off legacy software users in a bid to make a statement (and a VERY UNPOPULAR one).
I had recently brought my aging iMac back from the dead using Catalina, and USUALLY I keep abreast of Mac developments, but this time I didn't. Which is a shame, because I still played Cities in Motion with the fan-made suspension rail mod and other additions often. My iMac will now be a media creation and server machine while I build my own AMD/MSI machine specifically for gaming. I'd advise that you and other Mac users do the same.
