Why has malaria only now started to develop resistance against artemisinin?
Selection pressure (1, 2). One way or the other, there were few enough people taking the drug that it didn't really "matter" to P. falciparum (the deadliest of the malaria-causing parasites) on a population level until recently.

According to the 2014 paper
Spread of Artemisinin Resistance in Plasmodium falciparum Malaria (3), the driver of Artemisinin resistance is a single point mutation (ie a change of a single DNA base pair) in a single gene. The thing to keep in mind is that these single nucleotide mutations happen all the time. You yourself most likely have 3 or 4 of them relative to your parent's genomes. Thus, this particular Artemisinin resistance mutation probably has occurred in the past. 

However, as with most drug resistance mutations there is likely some fitness cost paid by the individual in which this mutation occurs. In environments without the drug, wild-type (ie non-mutant) parasites will outcompete the mutants, and the mutation will quickly disappear from the parasite population. It is only when the drug becomes commonplace that the mutant parasites gain the upper hand and the mutation spreads.