Why this issue happens?

Is there any way to fix it?

Now, you might find the answers in this post.

Microsoft blocks Windows 10 19H1 on older Mac devices

Indeed, a Microsoft support page has explained it.

However, it is there.

These devices have a compatibilty hold in place and they cannot update to Windows 10, version 1903.

Windows 10 Version 1903 vs. macOS Catalina

Whats new in Windows 10 latest version 1903?

What are the new features of macOS Catalina?

Windows 10 version 1903 vs. macOS Catalina, which one is better?

Then, you could go to check for updated Windows support software.

Likewise, in Mac OS, you gotta go toApple menu>App Store.

Then, you’re able to install all available updates on your setup.

You know the reason and you should not be so worried now.