I don't know this for a fact, but get the impression that Kwacka needed to get rid of their remaining stock of 250R parts and so did a limited run of Z250's to "clean out the parts bin" if you will.
I've inquired several times about a Z300 and so far it seems that's not in the pipeline, even though it would be a piece of cake for them to make it and I reckon it could be a good seller.
You're quite right that you can easily put Z250 bars on the Ninja 300. Or you can just get risers for the Ninja 300 bars, which ever you find more comfortable I guess.