I didn't think I'd ever find any reasons to dislike RWBY more than I did. I stand corrected.
The answer, while not specifically stated, is that it takes somebody able to hold the power present for it to be passed on; men cannot hold it, for one, and since the group knows that Rinoa was the best possible host for a Sorceress powers among the group (as Edea's powers went to her rather than Quistis or Selphie), then she'll still be the best possible host for Adel's powers too, provided she's present for Adel's defeat.
None of this is explicitly stated, but it's the only interpretations of the events that makes logical sense.
According to the timeline we have, Edea passed her Sorceress powers to Rinoa, and immediately, as in, before Rinoa had recovered from the shock of gaining those powers or Squall had time to react, Artemisia moved from possessing Edea to possessing Rinoa. It's the strongest clue we have that, yes, Artemisia will only possess other Sorceresses.
This also, while not explicitly stated, is something that we've already been shown by the game, it's just a matter of connecting dots.
1) Edea specifically said she gained the powers of two different Sorceresses, once when she was a kid and then again when she was a matron. So, multiple Sorceresses passing their powers to the same person happens (in fact it's what the group is trying to engineer with Adel and Rinoa), which would naturally lead to the total number of Sorceresses in the world decreasing over time as more and more power gets concentrated in fewer and fewer Sorceresses.
2) A Sorceress can be "forced" to pass on their power by being beaten down hard enough; again, with Rinoa as an example, we know that the person doing the beating down can be the one who inherit the power, too - that alone is a pretty strong motivation to hunt down Sorceresses, by the way, to steal their powers for yourself.
Anyway, the conclusion that is not stated, but easy to make, from these premises is that, as power became more and more concentrated in less and less Sorceresses due to point (1), they grew more and more powerful, and due to point (2), once one of them felt powerful enough, she could have hunted down and defeated by herself any Sorceress that was left in the world, especially if there were only very few. It seems likely that this is what Artemisia did, which left her as the last and most powerful Sorceress ever.
None of it is stated at all at any point, but the chain of logic is pretty solid and makes sense of everything that we've been told so far.
