Because the PLA forced themselves through several blockades before they were able to reach the square. It was at these blockades that the strongest resistance was met, and where the majority of the killing occurred.

We don't know for sure, but the order seems to be that [the PLA] have to get [to the square] by midnight. So by 10:00 p.m. they're getting desperate. They cannot fight their way through thousands of people with riot shields and billy clubs, so each of these columns coming into the city starts radioing into headquarters, asking for permission to go ahead at any cost. Finally that permission starts coming down sometime between 10:30 p.m. and 11:00 p.m.

The first rounds of fire catch everybody by surprise. The people in the streets don't expect this to happen. There are a couple of hospitals right near Muxidi, and the casualties start showing up within 10 or 15 minutes of the first round of gunfire. The casualties run very high because people didn't expect to be shot at with live ammunition. When they start firing, people say, "Oh, it's rubber bullets." Even after it becomes clear, even after they realize that the army is going to go ahead at any cost, people still pour into the streets. This is the amazing thing: People were just so angry, so furious at what was happening in their city that they were not going to step back and let the army do what it was doing. This is why the casualties from Muxidi on east towards Tiananmen Square were so high. This is the major military confrontation of the evening.

Source