Why there is no neurovascular drug-eluting stent?
Despite some superficial similarities, the design of coronary stents and neurovascular stents are quite different. Coronary stents are hard metal, made out of stainless steel. They are quite solid, and if you crush one, it will remain crushed. They are deployed by expanding a balloon at potentially very high pressures (10-20 atm). Coronary stents are relatively short, and are deployed in vessels that are relatively straight given the length of the stent.

Drug-eluting variants of coronary stents are additionally coated in a polymer (i.e., plastic) matrix that contains the active ingredient and releases ('elutes') it over time. The common commercially available drug-eluting stents release anti-proliferative agents to prevent neointimal growth.

The purpose is also somewhat different: you mention flow diverters, so you are likely thinking about the context of aneurysms, where they may be used alongside coils to seal off the aneurysm from the normal flow. They are not providing structural scaffolding to prevent vessel occlusion from atherosclerosis.

Neurovascular stents, including flow diverters, are often made of self-expanding metals like nitinol. They are more flexible, but try to keep their nominal shape. These stents are deployed by retracting a sheath and then they themselves pop out to their nominal diameter. Because of this different deployment strategy, they can also sometimes be retracted and repositioned. The way these stents deploy is a lot more gentle, and they are also typically longer than the coronary varieties. They also may be placed in vessels that can be accessed superficially, such as in the neck. A stainless steel stent in the neck would be a disaster: if you laid pressure on that part of the neck you could permanently collapse the stent. A self-expanding stent will bounce right back open.

Back to your actual question...

The actual purpose of coronary vs flow-diverting stents are different; the purpose of drug-eluting coronary stents, preventing neointimal hyperplasia, is not as relevant in the neurovascular context. Additionally, there are lots of good reasons to use self-expanding metals in the neurovasculature because of tortuous anatomy and superficial vessels in the case of the carotids. The hard plastics that are coated onto steel stents can't be easily placed onto self-expanding stents, so other techniques would need to be developed.

The article you read talking about the complex shape of cerebral arteries is in large part correct.