Certain generic elements or be it genre traits for a ninja anime normally involve things like feudal Japanese like setting, some sense of historical accuracy and a world in which sword play is most effective. A ninja based anime taken away from these elements and settings in my opinion really doesnt work, or ok it works very rarely. I’ll agree Nabari no Ou was an impressive anime which worked well with 21st centry ninja’s. However alot of the time it doesn’t.

Its like baking a cake but instead of something sweet you use something savoury. Sure you get the occasional masterpiece (the cheese cake), but alot of the time it ends up in a big mess of tasteless nudity, exaggerated fight scenes and story so predicable it makes Tite Kubo look genius and original.

What I’m saying is its bad to mix the two although only elements I’ll illustrate my point using genre. Take for instance Ultimate Muscle, it was Sci-Fi and sport. If you take sport out of its natural habitat its going to look silly and ridiculous, whether set in the future or in the past and by god did it look silly.

Or Ah My Buddha, ecchi and religion … yea lets not go there.

Legend of the Blue Wolves made a good attempt with its mecha yaoi (that was a joke).

Ok but i admit sometimes it works, and when it works, just like that awesome cheesecake its fantastic. Example being something like Cowboy Bebop with its Sci-Fi, Western jazz theme which almost redefined anime itself.

So in answer to your question why isnt there any Ninja Gun play around, i suppose no ones got the balls to pull it off yet.