It is very simple. The more usable/universal/backward compatible system, the less secure it will be.
So, Windows, in fact, cannot be secure. The same is true for the Internet.
The problem is that most people like usable, universal and backward compatible systems. That is why they constantly complain about security.
The Chromebook is very secure but not universal and usable only for the short spectrum of tasks. Ideal for browsing, social media, document editing, watching films, etc. But not for gaming, not for science applications, not for professional applications, not for programming, and not for many other tasks.
So, yes - Chomebook is very good and highly recommended for many average Joes, but not for all average users.
Furthermore, Windows OS has a great advantage to be with many people from the childhood, so they will not like to leave it for another system.