Because it makes it look like your going to piss your supporters off. There is really no other way to put it. You want to wait until your either voted out of office so you know it won't affect the outcome or you want to wait until you win (re)election so you have years to go before they can vote you out of office.
If politicians were half as concerned with the fate and direction of our country as they were about their own careers we would be running a multi-trillion dollar surplus with 3% unemployment. Instead we have a bunch of wimps who can't pass legislation because they know it will be unpopular because it screws over Americans.
Why is it so hard for some people to do the right thing? Is it not enough that after one term in the senate you get retirement and health care? That you have to work against the American people while you are there. Are these people just evil? ignorant? some fantastic combination of the two? Since you don't want to do the votes now I'm just going to assuming your plan is to screw us over.