The general public is never going to go for something so technical. ## MPG is an easy to understand term. Start telling people "Your new car has a ## gallon tank" they're going to stand there dumbfounded. They're not going to want to try to figure out how many miles that equates to, especially when there's so many variables involved (two cars with the same size tank can get very different mileage on that same tank).
That's not a slight on the average consumer either, it's just common sense IMO. If you asked me how far I am from New York City, I'd tell you 2 hours... telling you 125 miles, you'd PROBABLY be able to figure out how long that trip would be, but what about traffic? Are 50 of the miles on a 35mph road etc? 2 hours is a DIRECT answer to the question.
Basically, you should give people as close to the info as they want. They don't want to have to make calculations. So we say ## MPG and in-car displays shows ### miles to give people a good idea, at a glance, of what to expect.
So, just as no one is going to want to hear "you have a ## gallon gas tank", they're not going to want to hear "you have a ##kwh battery pack" -- what does that equate to in mileage? Sure to the technically savvy it MAY make more sense to talk density, but if the end game is mileage, then why not just talk mileage? After all, 2 packs with a similar density in 2 very different cars can produce very different end results right?
K.I.S.S.