The best time to plant is when the conditions are right for the planting. If you've never planted seeds in less than ideal conditions, it'll be hard to understand. At the time of Voyagers, the conditions were right in a way that only happens every 175ish years. Anything now and since would have been less than ideal conditions to the point the newly launched craft would not be as performant as Voyagers.

If we're waiting around on planetary alignments to do missions things we could do in decades will end up taking centuries. Speed isn't the only metric that matters; in the meantime we could be testing alternate forms of propulsion from lightsails to nuclear propulsion.