In Conte's first season, Chelsea rarely conceded quality chances/shots, so keeper wasn't called into action so much. Last season, Courtois started well, then become average/poor for a long time, until around April. After Barca game, his stock fell and all the exit rumors started to die, even Courtois said he wanted to stay. Then he had some good games towards the end of the season. Don't know if this was because he started caring again, or if his form just improved.

Two out of last three seasons at Chelsea were bad and that means plenty of opportunities for a goalkeeper to shine. But Courtois never really made anywhere near the impact you'd expect a world class keeper to make. Remember that this isn't a scenario where the keeper was playing in a top defense and rarely had a chance to show off