It's because the design was simple and cheap but effective. It's a distinctive, original design that was bought by everyone from farmers to the aristocracy.
Take the badge off that BYD and you would struggle to tell it apart from any other soulless boring car. In 5 years time BYD will have changed the design with nothing retained and still nothing distinctive about it.
This new 2CV on the other hand is very distinctive to the point where some will love it and some will hate it.
It's a good cheap, small car that will be easy to drive in cities. It's also "classless" car in the best sense.
Some people will want it regardless of how much money they have. It's the opposite of "try hard". You'd only buy a Seagull because you can't afford something better (from BYD or anyone else).
> No one is clamoring for an electric $50K VW Rabbit (VW Golf).
Actually the electric Golf (Rabbit) already exists, it's called an ID.3. It's about the 5th best selling electric car in Europe so it's, apparently, exactly what a lot of people want.