I know that Honda, Kawasaki and Yamaha have the knowledge, technology and resources to do it, but apparently they just don't have the balls to tackle the market until someone else proves it is large enough to be profitable.
I think that's Zero's and Brammo's job. If they can sell well in the thousands, the other brands might take more notice of the market. I think it will happen in a few years when most elmotos can easily pass the 100 mile threshold for less than $12K. That will make them competitive in the general market and not only in the niche market, which is not very interesting to big brands.
The best news would be if Polaris makes the Enertia Plus (and hopefully the Empulse) commonly available in its dealers. It would probably be even more successful, looking at Polaris general consumers, if they had the Engage and the Encite. With a mix like that, they could put some serious push in creating/expanding the market. One year later, if they do well enough, all the other big brands would jump in.
But this whole process will take a few years. None of the big brands seems to be in a rush...