They just need a Cerebro to extend their range. Or a psychic signal repeater.
With only three precogs they probably rely as much on the deterrant as actual intervention, so that’s not really an option for general use. It would drastically increase their workload, both from the attempted murderers who turn around and have another go and the would-be murderers who are no longer put off by the psychic police who will do bad things to you if you try something.
I loved Next- I keep hoping they’ll do a sequel. I mean, hell, the movie was really just one big setup for another movie.
I thought the implication was that there was a dirty side to the creation of the Precogs. They were the unfortunate by-products of some really awful drugs and the effects they had on pregnant women.
If you want Precrime to go nationwide, you need more Precogs. The Administration tells the populace that each Precog leads a glorious, happy life (complete with “bedroom, television, and weight room” as Bryan Ekers noted), but behind the scenes, they have to deliberately issue illicit drugs with terribly unpleasant side effects to otherwise innocent expectant mothers in the hopes of producing [del]the Kwizatz Haderach[/del] more Precogs.