The reason is because the decision to pit Hamilton was nonsensical. Lewis was leading at MONACO with so few laps left, and the team chose to bring him into the pits without waiting to see what the other drivers behind did, and without being certain that, even with a perfect pitstop, Lewis could be brought out and retain his lead when they did so.

Monaco is not like any other track on the circuit. It is insanely hard to pass there, and likewise, it is much easier to defend a position from a guy behind you. With that car, Lewis could have held off any threat from behind, even if his tires were hurting, because the guys behind were not on fresh rubber either. Mercedes did not just make a simple forgivable error in judgement by calling Lewis in - they made a MASSIVE mistake that even a rookie team should be rightly chastised for.

It was the team's job to inform Lewis of the situation and tell him that he did not need to come in just because his tires were going off. The pit team should have known that the guys behind would not have the rubber to pass him in the incredibly tight confines of Monaco. They should have told Lewis to make the road as wide as possible after restart to protect his lines, tough it out, and the victory would be his. Instead though, they did the opposite, even though any fool who could do basic math (given how long a pitstop takes out of a lap) could see beforehand that Hamilton would have a tough time pitting and retaining the lead.

The risk of pitting was nowhere near worth the reward of getting Lewis the fresh rubber. This was blatantly obvious to so many viewers, and the reaction of the Mercedes higher-ups (Toto and Niki for example) after the race shows that they know they screwed Lewis, and screwed him hard. Lewis did his job brilliantly all weekend, but his team running the show from the pits did not do theirs because they made a stupid strategic decision that even casual fans had to question the validity of.