Why are some Mac apps not available on AppStore? One example is CheatSheet. I searched it on the AppStore but couldn't find it.
Business Reasons

On macOS, applications can be sold outside of Apple's Mac App Store.

Many developers choose to sell their software through Apple's store in order to access the audience Apple provide.

However, there are numerous business reasons a developer might choose to avoid the Apple operated stores. These include control over pricing, product, and direct customer access:


  If you only sell your software in an app store and the store controls the contract, market place, and customer relationship, what exactly is your business built upon? You have one customer; the store. You are left at the whim of that store's policies. Are you really building your own business? Or are you building someone else's?


It is possible to sell Mac software both through Apple's Mac App Store and directly to customers outside the store.

On iOS, apps must be sold through Apple's App Store.