It was a unique period of interplanetary space travel where most projects were simple flybys - the first time for each planet. Because the goal was just to flyby, the secondary benefit is that the trajectory sends it outside the solar system.

Nowadays, most missions involve insertions into orbit around the target planet, therefore no secondary opportunity to send it outside the solar system. The notable exception is New Horizons, which was a Pluto flyby and will also eventually leave the solar system.