Bad coffee beans if not roasted to charcoal state taste even worse. Argument that that most of available coffee in VN is made from pretty bad beans, so roasters have no other way to roast it to that level.
That's it.
But a dark roast is easier to produce, and easier to produce consistently. If you take high quality beans, and low quality beans, and roast them both to a dark level (let's say "French" or "Italian" roast), they're going to taste approximately the same. Therefore if you're producing coffee at a larger scale and want to save money, you can use cheaper beans and roast them dark to mask the imperfections that come with the cheaper beans.
There are some truly incredible coffees out there and a well-executed light or light-medium roast will bring out those flavors beautifully, but the quality starts at the coffee farm. You can't light roast a low quality coffee bean and expect those same excellent flavors.
If you are in the last box, you cannot get to the first one, but you can still move one step up by burning it. Plus you can add a lot of milk and cream and then it is almost the same anyway.
Meanwhile, people that don't mind "burnt coffee" are the people that preferred coffee-based drinks from coffee shops. If I get a salted caramel mocha Frappuccino with two pumps of hazelnut - burnt beans are probably the only kind of beans I will be able to taste in that drink.