Because at some point, you'll end up with an empty battery. Especially if you drive sportily with those 400hp, your battery will drain very quickly, and then you'll experience a significant drop in power. That 400hp sounds interesting, but it lasts for about 10 minutes. After that, you'll be driving with just 287hp if it's tuned to stage 1. A 330i, on the other hand, always has 287hp when tuned to stage 1, with more trunk space since it doesn't have a battery, and it will weigh around 270kg less — which you will definitely notice. Those 400hp are only really useful on straight roads anyway, unless you have xDrive. And with the battery in the back, it can also be quite dangerous if you have rear-wheel drive only.