^Probably because coca leaves are Class A and because leaves are so bulky in any significant weight that they're not practical to conceal or smuggle. A kilo of leaves must be a couple of dustbin sacks full at least, so even on a small scale for peronsal use it would be risky to get em.
On a large scale, a drug smuggler would need a entire shipping container load or something to just to get a kilo or two of finished product.
Also, if busted, no doubt the entire weight of the leaves would be taken as the weight of cocaine (you know, like the entire weight of cut is counted even if purity is 5% etc), so you might get done for importing a hell of a lot more than the processed product you would get.