Why did GOP resist investigating 9/11 for more then a year?

Why did it take pressure from the families and public for an investigation?Why did it take public pressure for the de-clification of the PDR titled "BinLaden determined to strike inside US"?Why did Bush refuse to be sworn in and testify before the 9/11 commission?Why did he only agree to meet with the commissioners behind closed doors with Cheney in attendence?Why weren't they at least interviewed seprately?How can we believe men who won't stand before the public and swear to tell the truth?