They have failed to keep it secure for as long as I can remember. They are still able to sell it despite that, because people own Windows-only applications.
I would have agreed if you were talking about Windows XP but Microsoft has improved their security standing a lot since then. I think its a little disingenuous to call them unsecure more than other systems in the industry. Also the security environment has changed dramatically where monitoring and alerting are becoming key components. I know for a fact that Microsoft is investing heavily in those areas.