Because Reid is ready to play chicken with the troops on the ground:

Senate Majority Leader Harry Reid (news, bio, voting record) said Monday he will try to cut off funding for the Iraq war if President Bush rejects Congress’ proposal to set a deadline for ending combat.
The move is likely to intensify the Democrats’ rift with the administration, which already contends Democrats are putting troops at risk by setting deadlines.
“It’s time the self-appointed strategists on Capitol Hill understood a very simple concept: You cannot win a war if you tell the enemy you’re going to quit,” Vice President Dick Cheney said Monday at fundraising luncheon for Sen. Jeff Sessions, R-Ala.
[…]
Reid said Monday he was willing to take the tougher tack in light of Bush’s refusal to begin pulling out troops.
“If the president vetoes the supplemental appropriations bill and continues to resist changing course in Iraq, I will work to ensure this legislation receives a vote in the Senate in the next work period,” Reid said in a statement.

They don’t think they have enough votes to pass it but they believe that as violence continues more Senators will cave (in other words, they don’t trust the plan that they approved by an almost unanimous vote):

Reid’s proposal is unlikely to pass. But Democrats say they believe with each passing week – as the violence in Iraq continues and voters grow increasingly tired of the war – they pick up additional support.

And then there’s this:

The bill to cut off funds for the war would likely be introduced as standalone legislation and would not be tied to the supplemental spending bill, Manley said.

OK, so they’re not going to cut off funds in the bill that would fund the troops. Yes, that would make sense.
Not only do they have to pass this thing but Bush has to sign it into law, do they think he’ll do that? They will never get enough votes to override the veto and then they’re stuck funding the war because that’s their constitutional duty. I would think that’s the only way they can successfully get out from under the yoke of the anti-war movement. (“Hey, nothing we can do about it, just have to keep funding the war. Tried to defund the troops but Bush vetoed it. Oh well! We’ll try again next month.”)
Related post:
The Democrats are going to over play their hand