Presumably they don't want the great "success" of the program to detract from Obama's begging the Senate to pass $2 billion more. It's important to fund, then evaluate. It's the Obama way:
The Obama administration is refusing to release government records on its "cash-for-clunkers" rebate program that would substantiate-or undercut-White House claims of the program's success, even as the president presses the Senate for a quick vote for $2 billion to boost car sales.
Transportation Secretary Ray LaHood said Sunday the government would release electronic records about the program, and President Barack Obama has pledged greater transparency for his administration. But the Transportation Department, which has collected details about 157,000 rebate requests, won't release sales data that dealers provided showing how much U.S. car manufacturers are benefiting from the $1 billion initially pumped into the program.