[dzdt]

Received radio power decreases as the square of the distance. If the repeater started now, to be useful it would have to be similarly large to the big radio telescopes used to receive the signal. It would need to have the same speed as the probe it was following to keep up the signal. But the Voyagers each were massively accelerated by their giant planet encounters. Launching a much-more-massive repeater at much higher initial velocity than the original probes is nonsensical.

[jacobush]

Why would it have to be massive? In space, radio dishes can be unfolded mylar.

[lukeschlather]

The point is it would be massive relative to the probes, and it would also have to be moving faster than them to be useful anytime soon. And if you could launch something that massive going that fast, why bother making it a repeater. Just launch a new probe with a bigger antenna.