Why Apple's let to the iPhone 4s back to a 6.1.3 version?

🙂Actually, I have an iPhone 4s and 3 days ago I was looking some information about how to back from IOS 9.3.5 to IOS 6.1.3, my surprise was big when I saw that iTunes was given support again to install IOS 6.1.3. I took my iPhone as fast I can to start with the downgrade process and it worked! Finally, I have my iPhone 4s with IOS 6 again and I prefer than iOS 9 because I think that is more beautiful, I mean, the interface is great! but I have two problems

one: whatsapp doesn't gives support to IOS 6.1.3 only IOS 7.0 and later (that is horrible because I use whatsapp from my studies, my work and my social life)😐

two: the same problem with facebook messenger and snapchat

exist a way to have whatsapp again?😕