Because they are gutless wonders who don’t have the ability to stand up and fight against Congressional overreach:

Bush had warned congressional Democrats for weeks that he would reject a bill that includes a timetable for the withdrawal of U.S. troops from Iraq and billions of dollars in spending on items unrelated to the war. But the bill passed the Democratic-controlled House and Senate last week, and Democrats said it reflects the desire of most Americans to end an unpopular war.
The White House announced that Bush will meet with congressional leaders from both parties tomorrow afternoon in an effort to resolve differences over the war-funding bill. While expressing willingness to talk, Bush has given no indication of backing away from his demand for a “clean bill” with no withdrawal timetable.
In Congress today, meanwhile, Republican leaders adopted a strikingly different tone behind the public partisan jockeying, saying some agreement will have to be reached with Democrats and reached quickly. Senate Minority Whip Trent Lott (R-Miss.) said neither party can go into talks with absolute demands for what can and cannot be in the next bill.
“It’s time to stop laying down these guidelines, saying it’s got to be this, it’s got to be that,” Lott said.
He suggested Republicans will have to accept some of the non-military spending that they have been deriding for weeks as “pork.” And Bush will have to accept benchmarks for the Iraqi government that are tied to consequences, should they not be achieved. He suggested those consequences could be linked to non-military reconstruction aid, taking up a proposal floated last week by House Minority Whip Roy Blunt (R-Mo.). Blunt reiterated his support for benchmarks, saying any punishment for not meeting them should be a focal point of negotiations in the coming days.
“Some kind of compromise has to be worked out between the administration and the Democrats,” said Sen. George V. Voinovich (R-Ohio). “That’s how it’s done. Everybody holds their nose, and maybe a couple of times vomits, but you get it done.”

Voinovich and the rest of these gutless Senators need to understand that this isn’t some piece of legislation that they can craft with give and take and compromise, this is funding for our troops! Giving them what they need to fight the war that Congress authorized. This is giving the president the ability to fight and win a war and not tying his hands and holding up the funds to get the new troops in the theater.
Don’t these gutless Republicans understand that they have the power to fight back and support the war and their president? How the heck do these people expect us to swallow their cowardice time and time again? I’m getting really sick of it. Time to clean out the Senate with some primary challengers.