The reason, I believe, not very hard to find. Owing to the economic downturn in the recent times, people are slogging it out more than ever at their workplaces. Once at their home , they are so fagged out that they can think about doing little other than eat and sleep. Even sleep is a bitch as they worry about what surprise the new day is going to hit them right on their faces. Sex, has taken a back seat in these troubled times.
Given the lesser frequency of exercising their libido, and the lack of energy for doing even so, people find it difficult to accomodate the increased libido that this 'Wonder Drug' promises. Hence the decreased demand for it.