Because at that point, the circumstance of the race changed. It was no longer a normal 1-stop race while in clean air and dictating pace, it was a game of being in 4th, in traffic behind a Ferrari that had DRS as well while being boxed in from Vettel coming in from behind. With limited intel on what would happen on those softs with such high track temperatures, they [Hamilton] overheated the tire and lost performance on them while also having blistering and risk of a delaminating tire. This dictated the pace.
Before this happened, Hamilton/Mercedes showed very good pace relative to the Ferraris. It was only when the tires overheated, that they were forced to manage more aggressively (without much success because it was too late). Back to my 1-2 non-VSC scenario and that would have not happened or been way better manageable because they had a significant margin ahead. As said, the VSC and not pitting was a game-changer.
Again; On the SS, the Mercedes was the car setting the pace, even vs. the Ferraris when they put on fresh softs during the VSC phase. Mercedes still pitted because they wanted to stay ahead of Vettel and knew they would likely lose position anyway. Also under normal circumstances, overtaking a very similar paced car on such a track would have been highly unlikely, as was pretty much evident by Kimi on Ricciardo or the other way around.