There are several reasons

• The car got lighter as the fuel was consumed throughout the race
• It was raining earlier which made the track wet and led to lower grip. Near the end of the race, the track was dryer (still wet but not nearly as wet as earlier in the race) which meant the drivers had more grip.
• The tyres didnt wear much because the rain dropped the track temperature. This meant that the grip increase from the track getting drier was more than the grip lost due to tyre wear allowing the cars to go faster.

Less fuel in the cars is usually the main one (i.e. they're lighter), could also be drier conditions and more rubber on the track.