Wars are always about resources and especially energy.
I found this on a website.
France produces 85% of its electric from nuclear and hence uses approximately 10,500 tonnes of uranium each year. About 45% of that total uranium is mined in Canada and 32% is mined in Niger (a neighbor of Mali) by Areva, a company whose major stockholder is the French government.
For similar reasons, the US and other Western powers also have interest in North and West Africa - a major source of valuable light sweet crude oil. Militant Islam is seen as a threat to the control of those energy sources.