I think the reason why the CR-Z isn't eligible is because it's not a purely electric vehicle - whilst there is a small motor on board the Honda, most of the propulsive force actually comes from a 1.5 petrol engine.
In comparison, the Honda Insight and Toyota Prius have been engineered (in-game, at least) as range extender hybrids - whilst both have internal combustion engines, none of their horsepowers go straight to the driven wheels: instead, they're used to recharge the batteries and support the 'leccy motor.
There's always the chance I'm wrong, and it could be down to simply being a glitch or oversight by Polyphony's programmers. But that's why I reckon the Insight and Prius (and Fisker Karma, for that matter) are classified as 'electric cars' in Gran Turismo 6, and the Honda CR-Z isn't.