In simple terms GM works like this; it has a global brand which is Chevrolet and then it has regional brands, [-]like Holden down under[/-], Vauxhall in the UK, Opel in mainland Europe and Deawoo in Korea - all the regional brands are sold along side the global brand. Obviously some are just redabged products but in the case of Opel for example it does have competing products, same goes for Isuzu.
Ten years ago GMSA tried to align us with the global brand which is Chevrolet. This meant Opel products would be phased out and they assumed people with GM products would simply opt for other GM products. It did not work out that well and most people that had Opel bought another brand instead of Chevrolet.
Currently GMSA is trying to position Opel as their premium brand, that is why we can have the more upmarket Adam but not the Spark/Picanto rivalling Karl. For GMSA to bring in an Opel product it needs to fit their premium product bracket. This is also why most late model Opels all have VIN numbers starting with W0, they are imported from Germany and not built locally.