They did as soon as they came withing their reach. You can't make a successful liberation attack of a POW/Concentration camp when it's hundreds of miles behind enemy lines, especially not with 1940's military technology.
Because we needed our soldiers for the front lines and if we went straight for the camps - they probably would of just machine gunned all the prisoners into pits. We would of risked more innocent lives.
Also many camps were behind enemy lines - we would of have to send in special troops of soldiers to take on the soldiers at the camps - potentially dangerous - we could of had whole units of soldiers captured or killed.
Why would the Allies want to attack the concentration camps when most of them were nowhere near the front lines. It wasn't until the Allies had crossed into Germany that they found the concentration camps, and the stories which emanated. There may have been knowledge prior to the freeing of the prisoners but it was pretty well kept a secret. The primary objective of WW II was to defeat Hitler and Germany and the Axis powers.
They couldn't attack on land because they were far behind enemy lines, the only way to attack was bombing raids and all the prisoners were in the concentration camps, they should have bombed the railways leading to the camps though.
But as soon as they reached them they did liberate them.