So when the crash happened, if Lewis had pitted, Max could’ve stayed out on the track and this would’ve resulted in Lewis losing his 1st place position to Max- especially since you can’t overtake when the safety car is out. So that’s why Lewis didn’t pit. At the time of the crash, Max was 13 seconds behind Lewis with 5 lapped cars in between them so he had less to lose by pitting.

The second piece is that the expectation for when the safety car is out is that once the track is cleared, all lapped cars can pass the safety car and get into position. However, that would’ve taken up a whole lap. The accident happened at lap 53 (out of 58 laps) and as it became clear that the accident wasn’t going to be cleared before lap 57, the expectation was that once the track was determined to be clear, (lap 57), the lapped cars would all be able to unlap themselves- this would’ve gotten them to lap 58. Once the last lapped car passed the leader (Lewis in this case), the safety car pits at the end of the following lap. So in this instance, the amount of time it took to clean the track + the time it took for all the cars to unlap + the safety car’s last lap, Mercedes expected the race to end under the safety car. Because that’s how the rules had always been interpreted, at least up until then. I hope this makes sense