I suspect that BMW didn't go "crazy enough" when they built the i8 in terms of performance given the look and other compromises that must be made if you actually own one.
Exotic car buyers will put up with major inconveniences like not being able to enter or exit the car easily, no storage space, poor visibility, expensive repairs and maintenance etc., IF that exotic car does 2 things exceptionally well - draw attention and scare the pants off of you (and maybe a passenger) when you decide to really get on it.
The i8 did draw attention due to its looks, and while it wasn't slow... it just wasn't fast enough to impress the kind of buyer comparing it to anything remotely similar. So then, if you bought one you'd be left with a highly impractical car that draws lots of attention but gets boat raced in any sort of acceleration measurement by most other "exotic" cars of similar age and also by almost every Tesla Model S... and if you bought the Model S (for example) you got 10 times the electric range of an i8 AND better acceleration AND a trunk that holds actual stuff AND the ability for you and passengers to get in to and out of the car without resorting to basically rolling onto the pavement at some point in the process of getting in or out.
If the price dipped another ~$20-$30K it might actually be a fun car to own because it's so different than anything else ever made, but even at $60K for a used one I wouldn't want to put up with all the inconveniences and the "looks faster than it really is" stuff just to be seen in one or to drive one.