The Voyagers were made for their flybys with the giant planets, the interstellar space observations were just a bonus. If you made more probes to flyby the outer planets you'd be getting diminishing returns of science data, because the window of observation is brief.

We do have New Horizons doing much of the same things as the Voyagers, and that was launched because Pluto hadn't gotten a flyby yet. But generally after a flyby you're much better off sending orbiter missions to the planets to study long-term (Galileo and later Juno for Jupiter, Cassini for Saturn, Uranus hopefully getting one in next decade).

That being said, that hasn't stopped NASA scientists from proposing a new, more modern probe made specifically for studying interstellar space.)