MD900:
Not only is it a question of cost, but light helicopters haven't got too much spare payload for a TCAS system - I would think however that a twin turbojet pointy nosed thing would have a few kilos spare, and if not then a few less pies for the drivers.
But of course that means a 5 year procurement cycle, spending taxpayers' money on something useful instead of pensions for civil servants . . .