The bridge is a large map, though relatively constraining to the assault team. The combination of large size and thick fog makes distant target aquisition hard. In theory the defence team should win every time as there are severe chokepoints on this map where the assaulters must pass - it is relatively easy for the defenders to cover these with any weapon that comes to hand. The defenders can just take cover and wait, picking off attackers with rifles and grenades when they appear.
However, if the defence team is over eager and approaches the attackers then the opposite holds true. This seems to happen quite often and this factor generally stops this map being too biased towards the defenders.