They weren't charged with murder because the poor train driver took more than a year to die of his injuries. My father had to visit him (because of his job) and is firmly of the opinion that everyone forgets that Biggs et al are basically murderers who got away with it and were only sentenced for robbery.
If Biggs had served his time, that would be one thing. But he hasn't. He chose to run away, he spent decades sticking two fingers up to the criminal justice system, showing off about how he'd got away with hit, and only crawled back here when he needed the NHS.
Having said all that, he's an old, sick man (although clearly not as sick as his son claimed he was three years ago) so there are clearly potential grounds for compassionate release. But those who know a lot more about the details than me or any other observer have chosen not to release him - presumably they have good reasons.