OK, here's a possible reason. People's frequent normal use case for the majority of the car's time has the battery meter near full or in the upper levels most of the time, right? Charging to 90%, right? It goes down when driving it, but gets refilled fairly soon and sits mostly full again for many hours. People don't let the car sit near 0% for long. Now with the unusable locked out portion being at the top, this does effectively put that so-called "90%" on the display down lower, like 80%. Better for the battery, being a little closer to midpoint, but still higher than ideal. With your proposal, that would move that usable portion of the battery up higher, because you're leaving more underneath it, so people's normal "90%" marker instead of being truly 80% would be getting more like 86%. So it would be pushing the most frequently used state of charge higher, which isn't really the better thing for the battery.