As per T261407#7088136, a significant number of task creation events did not get processed (hopefully a one-time problem). When a task is stored in the DB, that disqualifies it from being generated again - the assumption is that there is an EventGate event somewhere in the pipeline; if we emit another event, there might be a few hours between the first event arriving (and thus the task becoming available) and the second event arriving, if the task gets done or invalidated during that time, the second event will bring the index and DB out of sync.
That means all articles for which the EventGate event got lost are permanently disqualified from becoming tasks. (Or semi-permanently - an edit to the article will clear the DB record.) On wikis where the number of valid task candidates is not that huge, this is can become a problem. There should be some way fix such pages, either automatically in refreshLinkRecommendations.php or manually in some maintenance script. (fixLinkRecommendationData.php would do it, but it's barred from running in production.)