Adding features is hard work, risky, expensive, and sometimes counterproductive (!).
It's hard work, because you need to change an existing, working piece of software with new code, and deploy it. Including possible database changes.
It's risky. Deploying a new version of your software might introduce nasty bugs, huge usability problems, or you might change the way things worked before, actually displeasing a lot of users.
It's expensive. Programmers aren't cheap. Then you need testing, QA, ticket triage... all things that cost a lot of time, and money.
Counterproductive. Your new features might only benefit a tiny portion of your userbase, and might not attract new customers. They might break your existing model. And so on.
Once you've reached a good feature set, doing continuous development is more of a life choice, rather than a business decision that makes sense.