There are space probes that flew past asteroids, some came close to it, some orbited and some landed on it and took samples. Those asteroids belonged in the main belt. Now, Lucy will become one of the two space probes that is set to explore the Jupiter Trojans. It was planned on 2017 and is set to launch on October 2021. But why it took so long to plan a mission for Jupiter Trojans? Most of the previous asteroid mission were planned and executed in the early years of 21st century and even flybys were done by Galileo and Cassini. The success rate of asteroid flyby and exploration missions were quite high including asteroid mining missions considering how difficult those missions were. So, why exploration of Jupiter Trojans were not planned in those previous missions? Is it due to their complex orbits? Or due to financial reasons?
Question: Why it took so long to plan a space exploration mission for Trojan asteroids?