Because he had earlier on admitted LTA's oversight for the failed SimplyGo project, and with so many accidents and happenings since he took over, like the CB ERP 2.0 ranjiao OBU unit nonsense, he could not take the pressure already and cannot afford admitting another mistake else his performance Bonus will definitely be affected (see table below), we are talking about the loss of hundreds of thousands of $$$ dollars, LOL.
This is the sad state of affairs in SG politics by PAP. The ministers will selectively choose cases to admit mistake while creating a smokescreen for others to save their own asses NBCB.