The App Release Notes pages.
For example there is no way to run e.g. v1.2.3 of the app on Jira 6.x since the Apps are developed against certain Jira APIs that change very much over time making apps incompatible. The effort of making apps backwards compatible is very high, and brings a high potential of errors.
If you want to use the latest app version you need to upgrade to the latest version of Jira.
I always try to be as fast as I can to provide a version compatible to the latest Jira version. But since there are from time to time big API changes that force me to reimplement certain GUI features or under-the-hood features this might take its time.