The reason I think is that GM is multinational. And while Buick is not doing well in the US, it’s doing very well in China (80%+ of sales of the brand is in China).
During the 2009 bankruptcy government bailout the politicians involved forced GM to trim their portfolio to 4 brands worldwide. They had to pick between Pontiac and Buick. Since Buick had a strong reputation in the rapidly growing Chinese market it was an easy decision to save it and kill Pontiac. At that time the EV market barely existed.
I could see them trimming down the number of brands -- too many dealers and redundant models -- in ONE national market, but that seems like it makes little sense on an international basis. You'd want to pick the brands in each individual market that are healthiest. I could see shuttering Buick US and keeping Buick CN alive, unless there's something weird about the marketing-- the Chinese market only buys Buicks because they're the same cars available in the US-- which seems unlikely (at least, I'd expect regionalization)
Considering Ford used to ship vans to the US with passenger seats, then unbolt the seats and send them back, for tax benefits, saying "we bolted a Buick logo on the ones going to China and a Pontiac one for domestic markets" is trifling in comparison.
I wonder if they'll eventually reinvent Saturn as an EV brand. Hummer has specific connotations, and those don't mesh with a consumer saying "I'm cross-shopping the Model 3, the Ioniq 6, and the Prius Prime". I figure of their classic brands, Saturn is the one that screamed "I'm innovative and a bit different from traditional GM".
> the Chinese market only buys Buicks because they're the same cars available in the US-- which seems unlikely (at least, I'd expect regionalization)
It's a historical accident. Buicks were the car of choice for party members and government officials in the postwar reconstruction boom so it became the prestigious brand when GM entered the Chinese market in the 1990s via a joint venture with SAIC.
GM tried to sell the Hummer brand during the bankruptcy but ended up just discontinuing it. They recently relaunched Hummer to make EV trucks.
Pontiac had no brand equity at the time due to many years of mismanagement and it was unknown outside North America. I can't imagine that any other company would have paid much for it.
