Because gross negligence infers no doubt whatsoever, it was the necessary judgement at the time to prevent public disquiet at what would have to have been regarded as a suspicious crash unless it was presented that there was no doubt whatsoever that it was just a case of pilot error.
This means that this judgement is not going to be lightly reversed without proof of an alternative cause at least until the last embers of resistance to reform in NI are extinguished.
Get it yet?
The airworthiness argument isn't going to do it.