It really wasn't. MacOS classic was full of vulnerabilities as was OS/2 and Linux up through 2004. Windows dominated because it was the biggest ecosystem.
What made Windows easy to exploit was that it enabled a bunch of network services by default. I don't know about MacOS, but Linux disabled network services by default and generally had a better grasp of network security such as requiring authentication for services (e.g. compare telnet and ssh).
Also, Windows had the ridiculous default of immediately running things when a user put in a CD or USB stick - that behaviour led to many infections and is obviously a stupid default option.
I'm not even going to mention the old Windows design of everyone running with admin privileges on their desktop.
What made Windows easy to exploit was that it enabled a bunch of network services by default. I don't know about MacOS, but Linux disabled network services by default and generally had a better grasp of network security such as requiring authentication for services (e.g. compare telnet and ssh).
Also, Windows had the ridiculous default of immediately running things when a user put in a CD or USB stick - that behaviour led to many infections and is obviously a stupid default option.
I'm not even going to mention the old Windows design of everyone running with admin privileges on their desktop.