I imagine that a lot of people login with their Strava-account on this website to check out the service, but forget about it shortly afterwards.
With the current constraints in terms of API limits it makes sense to “archive” users that have not logged in on the site for a considerable time (for example: 1 year). Activities of such archived users can be treated with lower priority, or even just be skipped for processing until the next time the user visits the website.