[[From
here]]
It was the same area where they had fought the strange woman previously, so Wesker was cautious. So far he hadn't run into trouble in the same area twice, though he had noticed monster remains in various areas. He had yet to notice any particular pattern in the deployment of monsters, but he couldn't monitor the entire facility.
This particular area was also slightly more hazardous to fight in because of the railing and the danger of being thrown over it by a strong enough enemy. The noise of breaking down a door would attract the attention of whatever was in the area, so he wanted to make absolutely certain there were no enemies waiting in the shadows to surprise them.
Unfortunately, with Relena's light on, there was very little chance of noticing any enemy before the enemy noticed them.