This is a follow-up question to this: How can crones minimize risks of exposure in a human trafficking enterprise?
Mana is life energy of the soul used in conducting spells. Crones are renegade witches that steal mana from other human beings through a ritual that ends up killing the subject. Through this method, the crone increases their internal supply and grows in strength. Unfortunately, this turns them into an addict. The more they use this method, the more they want. Now you would think that with a planet of billions, it would be easy to secure a victim to steal mana from. Poverty stricken neighborhoods and war torn places would be prime hunting grounds for people that won't be missed. However, the situation is more complicated. Stealing mana from any random individual results in a low payoff, as the crone cannot feed on most of that mana. Even a person born with an immense amount of mana would not be able to sustain a crone due to its minimal return.
A seasoned witch, who has had training and practice in the use of mana, would be of much higher stock and value, a prime target for a crone. Although a witch may have a very low amount of mana available to feed off of, it would sustain a crone far better than a non-mage. These candidates are usually well-known and established individuals in their profession, or have received some form of recognition in the world. This makes them difficult to secure due to their public standing, and puts the crone in more danger due to their potential victim's ability to fight back with magic.
Why would extracting mana from a magic user be more successful for a crone?