Really disappointing that this is top comment tbh.
[1] https://sci.esa.int/web/juice/-/58815-juices-journey-to-jupi...
Mass. Small payload fast. Or large payload slower. (Also flyby vs. orbital insertion. If you’re going fast, you don’t get to loiter.)
The mass of the fuel spent on breaking could be spent on other things, like extra instruments, or fuel for thrusters so you can orient more often and stay in orbit longer. For example, the Galileo[1], Cassini[2] and Dawn[3] missions ended primarily due to the probes running out of fuel. Perhaps not the best examples since they all had quite long runs, but still, more thruster fuel would probably have meant even longer missions.
[1]: https://solarsystem.nasa.gov/missions/galileo/timeline/#jour...
[2]: https://solarsystem.nasa.gov/missions/cassini/mission/grand-...
[3]: https://www.jpl.nasa.gov/news/nasas-dawn-mission-to-asteroid...