The drug you're referring to, known as flibanserin, is very different than sildenfail (Viagra) in structure and effect.

Viagra works for sexual dysfunction by allowing blood vessels to remain dilated in the erectile tissue of the penis in response to sexual stimulation.

Flibanserin is proposed to work on the serotonin and dopamine systems in the brain 1,2 (interestingly, study 1 done on male rats) and "may improve the balance between these neurotransmitter systems". So, it seems to be working in a pseudo-anti-depressant manner.

So one treats the physiological methods of arousal (Viagra) while the other (flibanserin) may help in a more neurological or psychiatric sense.

Anyhow, the clinical trials found no sexual effect in men 3 and no improvement in mood with either men or women, but increased sexual desire in women. So, getting to your question: is there a reason why the same effect wouldn't be seen in men? Well, I'm going to have to cop out a bit and just say that there can be very significant sex differences when it comes to the brain and the way drugs affect our brains -- and this dopamine/serotonin interaction in males 4 and females 5 may be more important for female desire and less so for male desire.