While maybe not ideal for the end user (choice is good), BMW usually works with the tire manufacturers to customize the tires for their vehicles. This is true for mainstream, 'common' brand names and models. To tell if one of those is designed for BMW (special tweaks), they mold a 5-pointed star on the sidewall. Often, you can buy the same tire with and without that. It's recommended to buy it with that star. The differences may be subtle.
FWIW, it's not uncommon for tire manufacturers to make special tires for a specific model. There needs to be some minimum volume to make it economical for a tire manufacturer. My guess is that the i3's volume just didn't support some other one taking up the challenge.
Now, why the tires available for the i3 are the way they are: maximizing range. This was more important on the original model, since battery capacity was relatively low.
Nokian does make a winter tire designed for the i3. I have them on another vehicle, and have found them to be excellent. I think, better than the Bridgestone version, although they are quite popular for their winter tire series.
While you may be able to substitute another tire on the i3 with spacers and tweaking, you will change the characteristics. That may be good or bad, depending on your needs and desires.
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)