They were going full speed ahead (22kts) and it was at night. In a known iceberg area, this was outright negligence. This was 1912, there was no radar, only a very cold lookout shivering in a crowsnest communicating by voice tube. With the size of the berg, taken with the speed and size of the ship, she just didn't turn fast enough to avoid it.

The First Officer also ordered the rudder hard to starboard, followed by ordering the propellers full astern. These commands don't typically work well together in shiphandling. Rudders require clean water washing over them to work well. By putting the propellers astern, you're effectively stealing water from the rudders, reducing the turning force while gaining negligible reduction in speed over the short term, especially from 22 knots. (Emergency evasion typically calls for the rudder hard-over and the engine full ahead) They hit the iceberg just 37 seconds after the lookout reported it. Additionally, given the steps necessary to get a steam plant to perform a crash stop, I doubt much was even gained prior to impact.

The irony is, ships are designed to hit things... head on. Had Titanic hit the iceberg head on, it would have crushed the bow and flooded the forward compartment, but she probably would have stayed afloat, possibly even limped home. By turning, it scrapped the iceberg along the side, popping rivets and gouging into several compartments which proved to be fatal. To make things worse, since the watertight compartments didn't go all the way to the main deck, the water simply flooded over the top of one and into the adjoining compartment.

http://www.encyclopedia-titanica.org/titanic-final-manoeuvre.html