New York Times buys Wordle. Let's use it as an example to dissect the properties of hits.

Limited

• Restrictions cause questioning and spark creativity.
• There is only one game per day. If something is less common, its value/worth goes higher.
• It's not a time consumer. It's just a simple word guess in a couple of seconds such that it respects your time and even if you're very busy, you can still keep up with it. It doesn't try to acquire more time from you.
• It gives a quick daily achievement satisfaction by overcoming its challenge.

Accessible

• It has a frictionless onboarding. It's simple so that getting started doesn't require anything special.
• It is available everywhere, every platform.

Shareable

• There is no customization or personalization. Everybody is working toward the same thing so people can communicate through it and they are connected.
• It's very easy to show what you have done.

This was the first viral and very successful game of the year but it won't be the last. I guess any program that has these properties, is very likely to be a next one. Good to keep in mind while developing and good luck!