Although we could debate the topic for a decade, there's no simple answer for West Africa's instability. There are, however, a bunch of insightful explanations that shed some light on how the region grew to be so restless. For the past half century, some sort of coup has derailed a West African roughly once a year, a symptom of deep seated unrest that diplomats and statesmen would love to remedy. The Atlantic Wire's own John Hudson laid out a few explanations that are worth reading, but we were well impressed by the commenters' take on the problem. Ben Lamb helped us visualize the issue:

I think it's interesting that you didn't mention the fact that, even on a continent of states whose borders were drawn arbitrarily by colonial powers, West Africa's boundaries are especially ridiculous. Most of those national boundaries were simply about having access to so many miles of coastline and the hinterland behind it, in hopes there would be something exploitable there. 

Because of that, they don't respect any sort of natural ethnolinguistic cleavages, nor in some cases any sort of reasonable geographic ones. Even if there weren't different pre-existing groups of people in the different regions of Mali, does that shape even look conducive to internal commerce as-is? Much less building a stable, integrated state-society relationship. I know that's a pretty broad generalization, too, but we shouldn't overlook it.