In the beginning, the development of online casinos was chiefly focused on Windows OS as the users of this operating system constituted over 95% of the entire market, which resulted in the 3% Mac users being ignored.
Things are now changing as nearly all casinos have a version that can be played by Mac players without having to download the software. As long as their Mac OS and Safari browser versions are compatible, players only need to connect to the internet and log into their gaming accounts.
As for download casinos for Mac, the best you can do for now is to download a Windows emulator or run Windows on a partition of your Mac computer. Software developers are not very keen on developing dedicated Mac software because they are very expensive, and the percentage of Mac users is still not very high. Many online casinos now offer online casino games for Mac users, so look for a Mac-friendly online casino before taking extreme measures.