There is a possibility that the pilot decided to continue on oxygen, but civilian passengers and many civilian crew are not used to being on oxygen for extended periods.
Emergency passenger oxygen is designed to keep them alive long enough to regain a viable altitude.
The real problem is that you won't realise that you have a problem in time to fix it.
Above 16,000 I'm watching my pulse oximeter.
Another possibility is that the pilot lost useful consciousness before he could effectively manage the problem. Older people and/or those with reduced lung function may need oxygen at lower altitudes.