Something I’ve noticed since learning shamisen and playing with a minyo group is that all of the shamisen playing vocalists I’ve seen are female. Even looking back at watching performances on YouTube, I can’t remember seeing a guy sing. Is there some sort of cultural thing where only women sing? Does it have something to do with the history of geisha as entertainers? Are there male and female singer specific songs and styles? I’m pretty sure I’m missing something as my understanding of Japanese culture and sample set of songs are limited, but the female domination in singing struck me as odd.
I’m not sure if it’s completely true, but there do seem to be more female traditional singers than men. That said, there are still plenty of male singers out there. Take a look at some video footage from the annual Tsugaru Big Five Contest for proof:
Two standouts for me:
- Takada Tooru singing Tsugaru Ohara Bushi (8:08)
- Yanagawa Toshihiro singing Tsugaru Yosare Bushi (15:06) (Also the eventual winner)
After rereading you post, I see you’re maybe talking about specifically shamisen-playing singers. In that case, I think it is more common nowadays for those people to be female, unless we’re talking about nagauta. I think for whatever reason hauta and jiuta are just more common as hobbies among women than men in modern Japan. Maybe men are too busy being soulless salarymen?
Origin of shamisen is one of those small questions with a really big answer. Yes, many women played shamisen, geisha ladies, in fact. But also blind monks/priests.
And there is a parallel tradition of shamisen played during bunraku puppetry shows, and this was always men. These men did not sing - they narrated in an exaggerated voice. So, you are correct - shamisen was mostly played by singing women.
That’s the short answer - I know how much I left out. Folks, feel free to pile on with more of the history. It’s pretty fascinating.
Come on, even soulless salarymen like to cut loose with some drinks and karaoke. 
It might just be a matter of culture and woman preferring to sing more than man as a hobby. That seems more common in the US culture as well.
I was wondering if it might also be a matter of the song lyrics being written from a woman’s perspective. That might also make it odd for a man to sing. It seems like an interesting and deep history.
Try “Traditional Japanese Music and Musical Instruments” by William P. Malm, Kodansha Press. It comes with a CD filled with some very cool and weird sounds. It answers questions like these in great detail. Plus Malm covers musical theory and scales, and as much as we hate playing scales, they are the core of mastery.