Well, specifically only Voyager 2 did to reach Neptune. Voyager 1 slingshot only Jupiter-to-Saturn, which is available much more often in a much larger set of planetary positions. Pioneer 11 was essentially rerouted on the fly to do that, and Cassini also flew a planned Jupiter-Saturn trajectory.
There's also a bit of a multiple endpoints situation in singling out the 4-out-of-4 giant planets scenario. That is indeed infrequent, but say a 3-out-of-4 lineup with either Uranus or Neptune, or 4-out-of-5 with Uranus and Pluto as the final two [dwarf] planets, also would have made for a fine and successful mission.
OTOH, we could even argue that we were unlucky that the giant planet alignment occured for that 1977 launch window. If it occurred this decade instead, we could send a much better spacecraft with the capabilities of Cassini or Messenger or New Horizons.
There's also a bit of a multiple endpoints situation in singling out the 4-out-of-4 giant planets scenario. That is indeed infrequent, but say a 3-out-of-4 lineup with either Uranus or Neptune, or 4-out-of-5 with Uranus and Pluto as the final two [dwarf] planets, also would have made for a fine and successful mission.
OTOH, we could even argue that we were unlucky that the giant planet alignment occured for that 1977 launch window. If it occurred this decade instead, we could send a much better spacecraft with the capabilities of Cassini or Messenger or New Horizons.