The Sun is a strong source of electromagnetic activity, and it wreaks havoc with the spacecraft's radio signal, essentially reducing the spacecraft's data rate to Earth to 0 for the two and a half weeks centered around conjunction.
Conjunction lowers the amount of data that can be returned to Earth.
However, Galileo still has roughly two years in which to investigate the Jovian system, so not being able to return data for 18 days out of those two years is not a serious difficulty.