Up until now, all probes targeting the four giant planets were launched in the last century.
The absence of follow-up missions is essentially due to obsolescence. As the situation changes, people are increasingly unwilling to invest in these exploration programs that span several decades.
Moreover, after the information explosion, people have gained a profound understanding of technological development: rather than spending 30 years to send a probe that would be extremely outdated by the time it arrives, it makes more sense to wait 25 years and then spend five years to send one that is a hundred times more powerful.
Especially after missing the once-in-nearly-two-hundred-years opportunity of the alignment of the four planets, waiting for new space technology became an inevitable choice.
The emergence of A100 was exactly the revolutionary space technology that the IAU needed.
