Android's hallmark feature is the ability to allow users to install apps even outside of the Play Store. This is useful if you need to test apps or if you want to avoid the hassle of submitting an app to the review process.
Despite the possibility of abuse, Google appears to be prohibiting the sideloading of applications that are older versions of Android.
This is because sometimes older software versions have exploits and vulnerabilities, which is why it is advised that users keep their apps and devices up to date. Hackers might try to exploit these vulnerabilities by creating applications that exploit older versions of Android, thus preventing the sideloading of such applications.
This will certainly not eliminate malware from being spread or installed entirely, but it might make it more difficult.