They have done plenty of cars which weren't iconic, but a bit sporty or interesting. quite a few of the cars they did could have been substituted for the equivalent Vauxhall, so they obviously had something against them.
Golf GTi or Pug 205 GTi could have been replaced by Astra GTE for example.
Also depends on your definition of iconic. It could be argued that Droopsnoot Firenza, Chevette HS, maybe even Mk3 Cavalier SRi, and certainly Lotus Carlton were iconic.
But that's the point, ask any car forum, outside of a Vauxhall one, to name an 80s hot hatch and the list will be populated by the Golf GTi, Pug 205 GTi, XR3i, RS Turbo, R5 GT Turbo and i would bet an MG Maestro/ Metro gets mentioned before many come up with the Astra
Name one from the 70's and it'll be the Lotus Cortina, Sunbeam Lotus, BMW 2002ti, Escort Mexico/ RS2000, Capri, Dolomite, P1800, or an American muscle car, most wont even think about a Vauxhall, apart from the odd rally fan who might mention the Opel Manta which isn't even a Vauxhall
Apart from a couple of years in the BTCC, one year when they were the only manufacturer, where is the motorsport heritage? How many Vauxhall F1 cars have their been? infact how many F1 teams have used a Vauxhall engine? How many times have Vauxhall won Le Mans. The last winning Rally car was an Opel badged machine
They have done an Opel, so it kind of tallies with what side of the branding brings the interest
There is no conspiracy or hatred, its purely the fact that a Vauxhall of any variety has very very little interest to the general public. Outside of the UK Vauxhall doesn't exist, every other make they have shown is pretty much a global brand and as its a global show, what are you going to do? Market forces etc etc