The deadliest form of malaria is Plasmodium falciparum, which mostly occurs in Africa. Doctors treat it with a combination of artemisinin and drugs such as doxycycline and chloroquine. Wormwood plants don't yield much of the drug, though, so there are perpetual shortages. That makes the combination therapy expensive, especially for people in the developing world, where malaria is common.
There's also another problem, which makes health workers everywhere nervous: malaria has developed resistance to a host of drugs developed to kill it. That's why combination therapies are used.
The plant has a lot of other chemicals in it besides artemisinin that might help defeat the parasite. Wormwood has flavonoids, for example, which have some antimalarial properties. But flavonoids are just one set of hundreds of chemicals in wormwood, a good part of which aren't aren't known yet. The complexity of the compounds in the plant can also make it tougher for the malaria parasite to evolve resistance.
Whether the findings can be translated to humans isn't clear yet. In the mice, the whole plant treatment reduced the number of parasites in the blood more than the purified drug, but the effect fell off after 72 hours. Odds are that if such a therapy were applied to humans it would require multiple doses. Even if that were the case, though, the fact that the plants can be grown locally and used as-is means the costs of treatment would come down significantly.