Why are some Mac apps not available on AppStore?

One example is CheatSheet. I searched it on the AppStore but couldn't find it.

There are a number of reasons developers may not want to go through the Mac App Store:

• Apple Developer account is required with paid annual subscription of $99
• Apple review process may block certain features or APIs
• Apple determines that the app does not follow other guidelines (ui, content, etc)
• Apple requires some features to be implemented or will not pass review (example, 64bit compatibility)
• Apps on the App Store run in Sandbox, and do not have access to other parts of the system except through Public APIs
• Pricing... Apple takes 30% of the purchase price
• Users are insulated from the developer... its like buying an electronic widget from a big box store but never turning in the warranty... the developer has no idea who is using their product.
• Time limited demos are not allowed (developers get around this by doing In-App Purchases for "Pro" features)
• They (the developer) has malicious intent (rare, but can happen... more often with free software though)

But there are also a number advantages for developers too:

• A great place to get exposure for your App
• Don't need to deal with purchases or refunds
• Support is limited to technical details and feature requests only
• Don't need to build in copy protection (serial numbers, activation, etc) because authorization is handled by the Mac App Store
• App is downloaded from Apples network, so don't need to worry about hosting bandwidth (an issue for popular software)

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.

Some developers don't like the App Store, and there's a wealth of reason why - Apple take too much money, Apple don't police the Store and stop the perpetual flood of cheap copies of decent apps, they don't like the restrictions placed on App Store stuff ...