Why aren't beast map bosses capturable?

Sorry if this was discussed somewhere else, I didn't find it. I'm just curious - if the unique monsters in the regular game can be captured for the Bestiary, why isn't the same the case with their map counterparts? Since we can capture, say, Q'uru, why can't we capture It That Fell which is essentially just his upgraded version? It would be great if they opened up new recipes, just like the campaign unique beasts do, but even without that - they could just count as beasts of their kind, or as duplicates of their campaign version?