Because, last time I checked, the STS is no longer operational and its replacement won't be ready before the end of the decade.
And its enemies would love to kill it thereby wasting money when they should be supporting it.
Its cost is going to kill it, not its enemies. The budget axes are going to start falling all over government, and NASA is going to have trouble maintaining a drawn-out development program for something that doesn't even have a payload when other branches are cutting essential services. It's going to face the same gauntlet as Ares and Constellation.
The Block 0 SLS can lift 70 tonnes and throws away both the solids and 3 RS-25D engines. The estimated cost of the Block 0 is $1.4 billion, which is $44,000 a pound to LEO, and its first launch is in 2018. Under the plan with the highest flight rate, it will then fly once a year till the launch of SLS #5 (which is upgraded to 100 or 130 tons), in 2021, and under the curent Senate authorization will fly once a year till 2026. That's nine flights for the next 14 years. Under the President's current budget we get 4 flights over the next 14 years. That's 930 metric tons delivered to LEO by 2026.
In the 1990's the Shuttle was making about 7 flights a year, and just counting the roughly 55,000 lbs in the cargo bay, that comes to 175 metric tons delivered to orbit, per year, which would've been 2450 metric tons delivered between now and 2026, instead of the SLS's 930 tons under the Senate plan, or the 280 tons under the President's budget.
According to the plans
the SLS isn't even as good at putting mass into orbit as the Shuttle was over the course of the next decade and a half, by a factor of about 3 to 10.
If that's the plan
the execution certainly won't exceed it.
That's just one reason why the SLS program gives me cause for concern.