Hassle and cost
I expect the idea is that people who've just bought a new car will want to drive it, thereby encouraging them to go places, which means they'll spend money. I'm not suggesting its the best use of a £6k tax cut mind, only that it's not just about the car, it's about the services the new owner uses.
The UK Government plug-in car grant scheme has been in place since 2011, when it was £5,000 (at that time it covered all ultra-low emission cars). It has been reduced over the years and is currently £3,000 and for electric cars only. See this on the Government website. I assume you won't get both grants simultaneously.
Electric cars aren't particularly popular because low and mid range model purchase price is about a third more than petrol, for far worse performance. They have a much smaller range, take problematic amounts of time to refuel during a journey and are considerably heavier. This estimate shows them as saving only about 10% total cost of ownership per mile.
Biting the hand that feeds IT
