> The only question is why [other manufacturers] didn't rat 
  > them out to the EPA.

Speculation: Ratting out VW carries some degree of risk that the public will become aware that there are better ways to test the emissions of vehicles. The public will then insist that the government legislate newer/better emissions testing, and the manufacturers will have an expensive new problem to solve. Better to keep quiet and let the existing, implemented and paid for (if flawed) process keep running.