The reason is because 6dof games tend to have really difficult skill expression around movement and positioning. With any 3d game it becomes a bit of a circle-

The reason is because 6dof games tend to have really difficult skill expression around movement and positioning. With any 3d game it becomes a bit of a circle-strafe fight. With space and 6dof games, it becomes a flight simulator fight, which is an intense genre. Additionally it removes some verticality from levels. IF EVERYTHING is accessible, it removes choices around taking the high ground / sneaking through the low ground. I agree they are cool games, but they have some quirks that are not everyone's cup of tea.