The difficulty with malaria is that its development cycle is long and it involves a number of factors. You cannot get malaria unless you are bitten by what we call an anopheles mosquito, a female mosquito.
However, in order to pass on malaria when it bites you, that anopheles mosquito must have ingested the blood of someone who already has malaria. The mosquito itself is not the carrier of the malaria parasite. The parasite has to come from an already-infected patient. So there are three factors: a sick person, an infected human, a female anopheles mosquito biting you, and a second person to whom the parasite will be passed. There is no direct transmission from person to person.
In that three-part chain, the parasite is transformed, both in the infected patient and in the mosquito, because, when that mosquito bites, the parasite is transformed inside the mosquito and when the mosquito passes it on to another sick person, there is a transformation as well. There are so many mutations between the three elements that it is difficult to find the appropriate sequence in order to identify the vaccine.