Well, i think the problem with Opel is that their pre-2004 models just were average cars. Not bad cars, but they didn't really have something that made you really want that car. Average performance, dull looks, dull handling, etc. No unique selling point(s). The "Ferrari wannabe" models (the Calibra and Manta) and quality problems around 1990 also didn't really do any good to their image
That Asta Coupe was actually designed by Bertone and tuned by Lotus. It didn't really have an Astra price tag though, it was quite expensive.