Sundance will tell you it's because of some grand deep state conspiracy to hide the truth. Sundance is an idiot. The FBI is under no obligation to provide Congress with any evidence in ongoing investigations. This tug of war between Congress and the President (the FBI works for him) has been going on since the beginning of our Republic. Congress always loses. They have no enforcement power, so there is no legitimate reason to give them anything.
For that matter, why on earth would the FBI hand over evidence to the people they are investigating before they are being prosecuted?