I had the same issue and started this thread which @Ignasi kindly merged with others.
Online private server, not an official server, but these were my results, and only after I started counting …
20 consecutive critter purges in a row at three locations belonging to my clan. Then, to my amazement, I got Starved Exiles. Two nights later, I got Pirates. The only change made (since we can do that on a private server) was backing off from 6 to 4 on the max level purge, but I blame RNG.
BTW, I had a level 6 (I suppose) purge of hyenas that included a named archer. Also, I never ‘work’ for a purge, I have fun, and they just happen as a results of the number of actions taken 
Regards
jim