The rare outer-planet alignment used by Voyager occurs about once every 175 years.
In the 1960s, JPL engineer Gary Flandro recognized that Jupiter, Saturn, Uranus, and Neptune would line up favorably in the late 1970s. A spacecraft launched during that window could pass one planet, gain speed through a gravity assist, and redirect toward the next one.
This geometry made the Voyager missions unusually efficient. Voyager 2 used successive flybys to visit all four giant outer planets: Jupiter, Saturn, Uranus, and Neptune. The spacecraft did not need to carry enough fuel to travel directly between each world; the planets' gravity supplied much of the trajectory-changing energy.
The alignment is sometimes described as a perfect straight line, but that is an oversimplification. What mattered was the planets' relative orbital geometry and the launch opportunities it created. Voyager 1 took a shorter route to Jupiter and Saturn, while Voyager 2 preserved the multi-planet Grand Tour path.