OSX has always been less secure than Windows as Windows has always been the most secure operating system.
What has made Windows so secure is that so many people use it so it is constantly under attack and so constantly being made more secure.
Other operating systems such as OSX have had so few people using them in the past that they haven’t been worth attacking. Now that Windows has become so secure that finding vulnerabilities is becoming extremely difficult the attackers are starting to consider the more vulnerable but less used OS.