Do you understand the concept of launch windows?
Launching a massive deep space probe like Cassini is a complicated thing. No available booster had sufficient energy to launch Cassini directly to Saturn. In order to reach Saturn, Cassini had to do several gravity assist maneuvers, including two fly bys of Venus, one of the Earth, and one of Jupiter.
Postponing the launch would've meant the planetary alignments needed for the gravity assists might not have been possible, or that an even more complicated trajectory would've been required. You can't just arbitrarily postpone these missions and easily recover. Delays add to costs, and cancellation would not have been impossible under the circumstances.
That's a high price to pay for scientific illiteracy.