Google, Apple and Microsoft have been making a lot of security-related changes in the OSes in the last few years. The result is that a lot of apps need updating or just no longer work.

It's not an issue for apps that are actively developed -- but it's devastating for side projects or open source apps that rely on volunteers.

Some changes (like new code signing requirements) might be possible to implement in a few days, but if you need to make bigger changes you are quickly looking at weeks of development time -- which is kinda hard sell for people who develop software in their free time.