Part of the decision to not fit individual air vents will be the fit-out cost to the company that specifies it. This is capital saving, but operational savings will also follow.
In some cabins, the jet is very far above your head and you have to have it turned up really high to 'reach' you. So that is another reason for not having it - because it just doesn't do the job.
I must state a preference FOR individual vents.