Because Opels are freaking good cars that GM is hellbent on making fail in the US market. The Opel Astra was once imported as the Saturn Astra. It wasn't even nearly as good of a vehicle as the current Astra, but it was still a cool 3 or 5 door hatch. Yet, GM only offers the garbage 1.8 NA engine instead of the 1.6T or 2.0T. Oh, and for some reason Saturn seemingly wasn't allowed to advertise anything except for the Aura, which probably was only even advertised because it was North American COTY. OK, so Chevy gets Saturn killed, 2nd gen Aura becomes Buick Regal. It's straight up Opel Insignia. Gorgeous, classy car, with a great engine choice on top. So, uh, how are people going to know about it? No ads for it really, but GM tries to shove the Malibu down our throats (remember, at this time it's the last gen TRASH Malibu). Then Buick gets the Verano, which is a surprising renaissance for the brand in it's goal of changing its perception in people's minds. Yet, GM allows the car to grow stale, then cancels it. Enclave is like 1000 years old, Envision is built in freaking China, and LaCrosse is...well it's a nice car, but let's be honest about the large car market. Encore is a great car (if you're into that kinda thing), but can the entire brand of Buick rest on a mini crossover?
Maybe the success of the Mokka/Encore in the US market will teach GM to double down on the whole Opel = Buick strategy, but somehow I doubt it. Can't let Buick step on any toes! Buick is dead in the US market, and its sad because their lineup was heading for greatness. Just imagine a Buick Astra with the 200hp 1.6 or the 250 ft-lb 1.6 Diesel. Buick could, of all brands, have the best hatchback in the US market.
I have no doubt in my mind that Chevy and Cadillac will one day succeed in killing US Buick.