What do you think has gone wrong for them?
I wouldn't ever consider one but that's due to an impression of mediocre build quality, cheap interiors and uncomfortable seats (my partner has confirmed the latter when she drove a service Insignia in the Reserves). I might be missing out on some cracking cars but I've just never seen them as a viable option, going back to when I was a kid.
I have owned 4 Vauxhalls, the current 2 I have owned for 13 and 8 years and mum is on her third Corsa. The build quality has been excellent on them all. Materials used are hard wearing (except the leather seats on 00's 'sporty' models). I have never had any issues with comfort nor has my mum (who is nearly 70).
Compare a Corsa B with a Fiesta MK3. The former was more stylish, had more rear legroom and had more up to date ohc engines, plus a Turbodiesel. The GSi looked just right for the time, Ford no longer had a comparable car.
Fast forward to now and I think its just a case of better alternatives being available.
As always, its seen as cool and trendy on here to despise them.