is because it contains a compound called icariin. Icaariin is a PDE inhibitor, and works similarly to all the various ED pills for men - they are all in the class of drugs called PDE5 inhibitors. They all work by increasing genital blood flow (works in women as well as in men).

Icariin is about 100-1000 times less potent (effective) than say Viagra, so if you need 5 mg of Vitamin V (a common dose overseas for women) then you need as much as 5 grams of pure icariin to get the same effect. That's a lot of gross-tasting powder to choke down.

Also, Icariin is non-selective, and inhibits other enzymes besides the desired PDE5 - so it may be more prone to side effects than Viagra.  

A drug called flibanserin was developed specifically to enhance female libido, and is approved in the US under the brand name Addyi.

An experimental drug under development by Palatin Technologies is the only known true "aphrodisiac" drug for both men and women. Called PT-141, it increases libido in men and women alike. However, it failed initial clinical trials because it causes a transient spike in blood pressure. Also, it has only been demonstrated to work in an injectable form, although theoretically a nasal spray might work as well. PT-141 can be bought over the internet.