Why? Because the president decided to go against the will of the people as expressed by their elected officials by vetoing any funding bill that included a set time for withdrawal from Iraq? Bush has veto'd two of said bills so to continually put forward anymore bills of of that nature would be the same as continually bashing your head against a brick wall. It's not going to do anything and your only going to end up being hurt if you continue.
They made their point. They've shown what the president thinks of the democratic process. Now it's time to move on.